fourcc: add RGB+x chromas

They are the common forms of VLC_CODEC_RGB32.

The default mask applied to VLC_CODEC_RGB32 corresponds to XRGB and XBGR on big and little endian respectively.

For now this MR only adds the chromas and the swscale translation. The RGB32 with default mask is automatically transformed into the matching chroma.

We can then gradually add the proper chromas usage where RGB32 is set/handled.