Rename lifetime Buffer<'a> to Buffer<'surface>

[madsmtm]

To make it a bit clearer that the buffer borrows from the surface.

[madsmtm]

We can also go with 's if you think 'surface is too verbose?

[notgull]

No issues with the verbosity here.

[MarijnS95]

I thought that we wanted to detach the Buffer from Surface entirely and let the user allocate+manage+present these (not sure which concrete issue), but that fundamentally requires a separate type as it is incompatible with the current "lock()"-like APIs.

#222 (comment) appears to be somewhat related, though with the caveat that I'd still let "the Context" allocate a backend-specific Buffer that's initially unowned/dissociated from a Surface (#306 (comment)).

[madsmtm]

I thought that we wanted to detach the Buffer from Surface entirely and let the user allocate+manage+present these (not sure which concrete issue), but that fundamentally requires a separate type as it is incompatible with the current "lock()"-like APIs.

Yeah, I'm not sure how / if we're going to square that circle, so I did this for now.