This is a small prerequisite for moving decoded YUV frames from
ImmutableBitmap into Media::VideoFrame. Once the frame owns the planes
directly, CPU consumers and the Skia upload path will share the same
YUVData object.
Those consumers need different representations. Skia expects high bit
depth YUVA pixmaps to be full-range 16-bit samples, while CPU conversion
paths must see the decoder-native samples and the original bit depth.
Keep YUVData in decoder-native form, and make the Skia expansion a
temporary copy owned by SkYUVAPixmaps. Add a regression test that checks
10-bit pixmaps are expanded for Skia without mutating the source planes
or changing YUVData::to_bitmap() output.
