Adds loads/stores (but not dst loads/gathers) for:
*fp16 single and two channel
*fp32 two channel
*normalized unsigned 8 bit two channel
*normalized unsigned 16 bit single and two channel
TODO: 4 channel unsigned normalized 16 bit.
MISSING: fp32 single channel. No matching current or future GrColorType
planned AFAIK.
Intent is to support all (noncompressed) GrColorType load/stores in
order to implement fallbacks for 3D API limitations on texture uploads
and render target readbacks (some of which require swizzling).
Also, can be used to support YUV<->RGB planar
splitters/joiners on CPU.
Bug: skia:8962
Change-Id: I258d5682a1f4025b31639a97b1a1a02077a2453f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/219999
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
217522c40d