mock: add an option to generate color bars for RGB

This can be useful to test the order of colors in RGB with mask.

The 15-bit and 16-bit values are currently written as Little-Endian to verify that's what Windows expects. It can be extended when we add hardcoded LE and BE chromas.

Used to validate !4253 (merged) with the Windows player.

Tested with:

• mock://video_track_count=1;length=6000000;video_colorbar=true;video_chroma=RV32
• mock://video_track_count=1;length=6000000;video_colorbar=true;video_chroma=RV24
• mock://video_track_count=1;length=6000000;video_colorbar=true;video_chroma=RV16
• mock://video_track_count=1;length=6000000;video_colorbar=true;video_chroma=RV15