fourcc: don't support RGB+x in OpenGL
XRGB was the default 32-bit RGB chroma with a mask. In some cases RGBA might be be supported, but it can't be handled locally from a XRGB source.
OpenGL SW interop used RGB+x as RGB+alpha but the alpha value will be used with undefined values. It might be possible to discard it inside the shaders, to avoid copies, but this is not the case now.
The handling of XRGB as RGBA seems bogus (and legacy code) anyway.
v5: remove RGBA fallback which is not needed by this MR since 38dd68f9
Edited by Steve Lhomme