Updated formatRGB.cfg to include RGBtoGBR conversion

It is suggested to include configuration for the RGBtoGBR conversion in formatRGB.cfg. At the moment that configuration is included in the "per-sequence" configuration files, which is confusing especially when operating VTM with sequences outside of the JVET's testsets. In those cases it is obvious that RGBtoGBR conversion should be used and formatRGB.cfg would be the natural place to include the required settings.

In addition to the suggested change, we might want to consider keeping the old formatRGB.cfg but renaming it formatGBR.cfg as the settings there are what one would expect to use for the GBR format (instead of the RGB format).