Enable transparent background support in our X11 EGL backend.

[nullgemm]

Currently, it is not possible to get a transparent background when using EGL with X11. This issue has been known since forever and a fix was already in development when this project started four years ago. At the time of writing, it is still being discussed.

Why is this still not fixed?

We need to wait for a fix to be merged in mesa. Discussion about it is happening here:
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9989

[nullgemm]

Mesa PR #9989 finally got approved last saturday after 11 years of development, so I implemented EGL transparency in 3eccc34. Until the next mesa release, it is possible to test this change easily under Arch using Laurent Carlier's mesa-git Pacman repo. For compatibility reasons, we will keep falling back to opaque contexts for the next few years in case people run old mesa versions.