Draft: core: add common R5G5B5 chromas without a mask

Similar to !4183 (closed) for 15-bit.

The Linux DRM has DRM_FORMAT_XRGB1555 and DRM_FORMAT_BGRX5551 that correspond to our chromas. But libavutil/swscale does't have them either. So we won't have any converter to other chromas understood by the rest of the code.

Draft on top of !4219 (closed) and !4224 (merged)

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v6: Draft on top of !4269 (closed)