Draft: core: add common RGB24 chromas without a mask

~~And remove the RGB24 without a mask since all the cases that were handled are now handled without a mask.

RV24 is mapped to VLC_CODEC_BGR24 which corresponded to the default mask on Little-Endian machines.~~

Draft on top of !4024 (merged) to make rebasing easier.

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v5:

• keep VLC_CODEC_RGB24M for now
• rebase on master to get the new interop_sw code
• rework some bogus assumption that the mask to memory order depends on endianness

v6:

• fix some WORDS_BIGENDIAN left
• handle the blend component shift per chroma with a function

v19: Draft on top of !4269 (closed)