Fix char types

[rsxrwscjpzdzwpxaujrr]

On my machine, I'm getting this error while trying to build:

Details

$ cmake ..
-- The CXX compiler identification is GNU 15.2.1
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found OpenGL: /usr/lib/libOpenGL.so
-- Found SFML 3.1.0 in /usr/lib/cmake/SFML
-- Found X11: /usr/include
-- Looking for XOpenDisplay in /usr/lib/libX11.so;/usr/lib/libXext.so
-- Looking for XOpenDisplay in /usr/lib/libX11.so;/usr/lib/libXext.so - found
-- Looking for gethostbyname
-- Looking for gethostbyname - found
-- Looking for connect
-- Looking for connect - found
-- Looking for remove
-- Looking for remove - found
-- Looking for shmat
-- Looking for shmat - found
-- Looking for IceConnectionNumber in ICE
-- Looking for IceConnectionNumber in ICE - found
-- Configuring done (1.7s)
-- Generating done (0.1s)
-- Build files have been written to: /home/rsxrw/projects/SFGUI/build
$ make -j2
[  1%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/Alignment.cpp.o
[  2%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/Adjustment.cpp.o
[  2%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/Bin.cpp.o
[  3%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/Box.cpp.o
[  4%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/Button.cpp.o
[  5%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/Canvas.cpp.o
[  6%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/CheckButton.cpp.o
[  6%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/ComboBox.cpp.o
[  7%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/Container.cpp.o
/home/rsxrw/projects/SFGUI/src/SFGUI/ComboBox.cpp: In member function ‘virtual void sfg::ComboBox::HandleMouseMoveEvent(int, int)’:
/home/rsxrw/projects/SFGUI/src/SFGUI/ComboBox.cpp:243:23: warning: conversion from ‘int’ to ‘float’ may change value [-Wconversion]
  243 |                 if( ( x > GetAllocation().position.x ) && ( x < GetAllocation().position.x + GetAllocation().size.x ) ) {
      |                       ^
/home/rsxrw/projects/SFGUI/src/SFGUI/ComboBox.cpp:243:61: warning: conversion from ‘int’ to ‘float’ may change value [-Wconversion]
  243 |                 if( ( x > GetAllocation().position.x ) && ( x < GetAllocation().position.x + GetAllocation().size.x ) ) {
      |                                                             ^
[  8%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/Context.cpp.o
[  9%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/DejaVuSansFont.cpp.o
[  9%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/Desktop.cpp.o
[ 10%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/Engine.cpp.o
/home/rsxrw/projects/SFGUI/src/SFGUI/Engine.cpp: In member function ‘sf::Vector2f sfg::Engine::GetTextStringMetrics(const std::u32string&, const sf::Font&, unsigned int) const’:
/home/rsxrw/projects/SFGUI/src/SFGUI/Engine.cpp:140:65: error: call of overloaded ‘getKerning(uint32_t&, const char32_t&, unsigned int&)’ is ambiguous
  140 |                 metrics.x += static_cast<float>( font.getKerning( previous_character, current_character, font_size ) );
      |                                                  ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/rsxrw/projects/SFGUI/src/SFGUI/Engine.cpp:140:65: note: there are 2 candidates
In file included from /home/rsxrw/projects/SFGUI/src/SFGUI/Engine.cpp:7:
/usr/include/SFML/Graphics/Font.hpp:302:111: note: candidate 1: ‘float sf::Font::getKerning(uint32_t, uint32_t, unsigned int, bool) const’
  302 |     [[deprecated("Use the getKerning(char32_t, char32_t, unsigned int, bool) overload")]] [[nodiscard]] float getKerning(
      |                                                                                                               ^~~~~~~~~~
/usr/include/SFML/Graphics/Font.hpp:325:25: note: candidate 2: ‘float sf::Font::getKerning(char32_t, char32_t, unsigned int, bool) const’
  325 |     [[nodiscard]] float getKerning(char32_t first, char32_t second, unsigned int characterSize, bool bold = false) const;
      |                         ^~~~~~~~~~
/home/rsxrw/projects/SFGUI/src/SFGUI/Engine.cpp: In member function ‘sf::Vector2f sfg::Engine::GetTextStringMetrics(const sf::String&, const sf::Font&, unsigned int) const’:
/home/rsxrw/projects/SFGUI/src/SFGUI/Engine.cpp:207:89: error: call of overloaded ‘getKerning(uint32_t&, const char32_t&, unsigned int&)’ is ambiguous
  207 |                                         metrics.x += static_cast<float>( font.getKerning( previous_character, current_character, font_size ) );
      |                                                                          ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/rsxrw/projects/SFGUI/src/SFGUI/Engine.cpp:207:89: note: there are 2 candidates
/usr/include/SFML/Graphics/Font.hpp:302:111: note: candidate 1: ‘float sf::Font::getKerning(uint32_t, uint32_t, unsigned int, bool) const’
  302 |     [[deprecated("Use the getKerning(char32_t, char32_t, unsigned int, bool) overload")]] [[nodiscard]] float getKerning(
      |                                                                                                               ^~~~~~~~~~
/usr/include/SFML/Graphics/Font.hpp:325:25: note: candidate 2: ‘float sf::Font::getKerning(char32_t, char32_t, unsigned int, bool) const’
  325 |     [[nodiscard]] float getKerning(char32_t first, char32_t second, unsigned int characterSize, bool bold = false) const;
      |                         ^~~~~~~~~~
[ 11%] Building CXX object CMakeFiles/SFGUI.dir/src/SFGUI/Engines/BREW.cpp.o
make[2]: *** [CMakeFiles/SFGUI.dir/build.make:247: CMakeFiles/SFGUI.dir/src/SFGUI/Engine.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [CMakeFiles/Makefile2:178: CMakeFiles/SFGUI.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

This simple fix fixes the problem. It's safe, as our current_character is also already char32_t, because we're getting the characters from an std::u32string, which is defined as std::basic_string<char32_t>.