Luminance textures are not supported on GL3 and as the textures are
accessed via a shader anyway it doesn't seem like it should make much
difference which component the single-component textures are in. Cogl
already has code to fake alpha textures via the texture swizzle
extension on GL3.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit ca1666860a325fa4d2362cdd38297d6281e997d8)
e46ad9a91e