This provides a way to convert YUV in fixed point (or pseudo-fixed
point; probably best to say "uint") to float, or rather coefficients
to perform such a conversion. Things like colorspace conversion is out
of scope, so this is simple and strictly per-component.
This is somewhat similar to mp_get_csp_mul(), but includes proper color
range expansion and correct chroma centering. The old function even
seems to have a bug, and assumes something about shifted range for full
range YCbCr, which is wrong.
vo_gpu should probably use the new function eventually.