xwayland: fix glamor es black windows

On some drivers (mesa included), when we request GBM_FORMAT_XRGB8888,
driver set surface storage format to GL_RGB8, which breaks gles2 rendering.
If we force set gbm_format to GBM_FORMAT_ARGB8888, then rendering
will happen and working. For correct colors, we need !924.

Fixes #1288
Fixes #1356

Signed-off-by: Konstantin <ria.freelander@gmail.com>
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