Support decoding images to multiple formats, depending on usage

Support decoding images to multiple formats, depending on usage

Our codec generator will now preserve any asked-for color space, and
convert the encoded data to that representation. Cacherator now
allows decoding an image to both legacy (nullptr color space), and
color-correct formats. In color-correct mode, we choose the best
decoded format, based on the original properties, and our backend's
capabilities. Preference is given to the native format, when it's
already texturable (sRGB 8888 or F16 linear). Otherwise, we prefer
linear F16, and fall back to sRGB when that's not an option.

Re-land (and fix) of:
https://skia-review.googlesource.com/c/4438/
https://skia-review.googlesource.com/c/4796/

BUG=skia:5907

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4838

Change-Id: I20ff972ffe1c7e6535ddc501e2a8ab8c246e4061
Reviewed-on: https://skia-review.googlesource.com/4838
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>