Add NewRGB() with float gamma to SkColorSpace public API

Necessary because PNGs like to specify their gamma this way.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3402

Change-Id: I399984d611db907b115b345df1afc88d39326fbb
Reviewed-on: https://skia-review.googlesource.com/3402
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
6 files changed