GLX 1.3 implies equivalent functionality, so this is safe to enable
unconditionally, and bindContext always takes both drawable and readable
arguments in any case. Mesa stopped exporting the __DRI_READ_DRAWABLE
extension in 8.0 (when the DRI1 drivers were removed) so this will
restore the extension string.
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>