Storing these was pointless, since they're only used briefly to provide
the info needed to upload the buffers to the GPU.
For BT.2020 coefficients, we can just say that the bit depth is 16 bits
since we're always bit replicating to that for Skia's shaders.
This saves us from having our own color conversion code, which was
taking up a fair amount of time in VideoDataProvider. With this change,
we should be able to play high resolution videos without interruptions
on machines where the CPU can keep up with decoding.
In order to make this change, ImmutableBitmap is now able to be
constructed with YUV data instead of an RBG bitmap. It holds onto a
YUVData instance that stores the buffers of image data, since Skia
itself doesn't take ownership of them.
In order to support greater than 8 bits of color depth, we normalize
the 10- or 12-bit color values into a 16-bit range.
Very minor change, which doesn't actually affect our output, since we
were already inputting and outputting microseconds, but it can't hurt
to give FFmpeg's decoder this information as well.
