WIP: renderer-native: Remove hard-coded selection of GBM_FORMAT_XRGB8888

And choose the first format matching our EGL requirements according to regular eglChooseConfig semantics. Which means choosing the highest color depth.

The reason why it was hard-coded to GBM_FORMAT_XRGB8888 originates from commit 712ec30c to fix #2 (closed) However the entire problem was later removed by commit 14c706e5. So we don't need to limit the GBM format any more.

In practice this means most systems should now get 30-bit color instead of 24-bit.

Performance considerations

For most drivers we would expect no change in memory bandwidth requirements since both the old and new formats are 4 bytes per pixel.

Compositing of native Wayland GL clients is made slightly faster and more efficient using 30-bit because they already default to XR30 or AR30 formats when possible. Only legacy Xwayland clients and most SHM clients still using 24-bit color will see slightly slower compositing.

Prerequisite: !1491 (merged)