egl_dri2/wayland: handle creating xrgb8888 images

When creating an EGLImage from a struct wl_buffer * this ensures
that we create an XRGB8888 image if the wayland buffer doesn't have an
alpha channel. To determine if a wl_buffer has a valid alpha channel
this patch adds an internal wayland_drm_buffer_has_alpha() function.

It's important to get the internal format for an EGLImage right so that
if a GL texture is later created from the image then the GL driver will
know if it should sample the alpha from the texture or flatten it to
a constant of 1.0.

This avoids needing fragment program workarounds in wayland compositors
to manually ignore the alpha component of textures created from wayland
buffers.

krh: Edited to use wl_buffer_get_format() instead of wl_buffer_has_alpha().

Reviewed-by: Kristian Høgsberg <[email protected]>

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