centralize default src/dst colorspace policy

We've long interpreted null source colorspaces as sRGB.

We've also been treating null destination colorspaces as legacy,
non-color-managed drawing.  Assigning dst=src in this situation
is the best way to express that.

I have reverted the changes to SkImageShader and SkBlitter_Sprite,
where we had very slightly different logic.

Change-Id: I90e5ab424aea534d83cfbecd282b6cf48b56d056
Reviewed-on: https://skia-review.googlesource.com/148996
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed