Draft: core: add common R5G6B5 chromas without a mask

And use the new chroma in place of VLC_CODEC_RGB16 where possible.

Tested the D3D11 and D3D9 variants and the swscale conversions.

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In v8 VLC_CODEC_RGB16 is removed and the VLC_CODEC_RGB565LE FourCC is "RV16" to minimize issues with existing code.

v10: make more generic blend mask usage as in !4225 (closed), Draft on top of !4219 (closed) and !4224 (merged)

v14: Draft on top of !4269 (closed)