Remove SkImageShader -> SkColorShader optimization.

Remove SkImageShader -> SkColorShader optimization.

It's turned out to be surprisingly subtle to make the optimization colorspace
correct (it's not today, but with this CL the affected images become colorspace
correct).

Before implementing a colorspace correct color shader, let's check we really need this.

Update: I should say, it's hard to do this optimization and make the resulting color shader be both colorspace correct and "legacy correct".  SkColorShader and SkColor4fShader are both colorspace correct.  It's just challenging to create one shader that's correct for both modes from an image.

Change-Id: I6593b6348175a10d7cbaaf3b531d7a7e2bf2f578
Reviewed-on: https://skia-review.googlesource.com/6548
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>