Support Float32 output from SkColorSpaceXform

Support Float32 output from SkColorSpaceXform

* Adds Float32 support to SkColorSpaceXform
* Changes API to allows clients to ask for F32, updates clients to
  new API
* Adds Sk4f_load4 and Sk4f_store4 to SkNx
* Make use of new xform in SkGr.cpp

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2339233003
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Committed: https://skia.googlesource.com/skia/+/43d6651111374b5d1e4ddd9030dcf079b448ec47
Review-Url: https://codereview.chromium.org/2339233003