Support encoding/decoding F16 without a color space

Change encode-srgb-* to exercise this code. The encoders
handle this just fine, so just remove those checks. The
decoders still rely on having a color space, so detect
and fix this inside SkCodec. Eventually I'd like to get
rid of the swizzler, use skcms for all format conversion,
etc... but that's a really big change, and this will let
us land other changes without breaking layout tests.

Bug: skia:8382
Change-Id: I8d85bf44ee3a287ad295515a745ded2ff5051417
Reviewed-on: https://skia-review.googlesource.com/156627
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
4 files changed