Fix DRI2 accelerated EXT_texture_from_pixmap with GL_RGB format.

Fix DRI2 accelerated EXT_texture_from_pixmap with GL_RGB format.

This requires upgrading the interface so that the argument to
glXBindTexImageEXT isn't just dropped on the floor.  Note that this only
fixes the accelerated path on Intel, as Mesa's texture format support is
missing x8r8g8b8 support (right now, GL_RGB textures get uploaded as a8r8gb8,
but in this case we're not doing the upload so we can't really work around it
that way).

Fixes bugs with compositors trying to use shaders that use alpha channels, on
windows without a valid alpha channel.  Bug #19910 and likely others as well.

Reviewed-by:	Ian Romanick <ian.d.romanick@intel.com>