dri_interface,egl,gallium: only expose RGBA visuals on Android

dri_interface,egl,gallium: only expose RGBA visuals on Android

X/GLX can't handle them. This removes almost 500 GLX visuals that were
incorrectly exposed.

Add an optional getCapability callback for querying what the loader can do.

I'm not splitting this patch, because it's already too small.

v2: also add the callback to __DRIimageLoaderExtension

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Cc: 17.2 <mesa-stable@lists.freedesktop.org>