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Only enabled for the DRI backends at the moment. In principle WGL/CGL
could support this - it's sort of implied by GL 3.0 support - but in
practice their implementations back GLX drawables with native drawables
(and not anonymous FBOs), so they would need either a corresponding
window system binding extension or significant implementation work.

v2: Require that the two screen numbers match, per v4 of spec.

Khronos: https://github.com/KhronosGroup/OpenGL-Registry/pull/102Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Eric Anholt <eric@anholt.net>