https://skia-review.googlesource.com/c/skia/+/255264

Bug: oss-fuzz:11040

Because we're sampling, the offset ends up the same as the width. Back
up to the left enough to fit the bytes we will write.

Change-Id: Ie476a0191b66c2322446b9c0922f630d6e971645
Reviewed-on: https://skia-review.googlesource.com/c/164262
Commit-Queue: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Leon Scroggins <scroggo@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>

Bug: oss-fuzz:11114

fSrcOffsetUnits is where we start sampling from the image. It is
computed as

  (sampleX / 2) * fSrcBPP

(ignoring fSrcOffset, which is 0 for a GIF with a subset frame).
sampleX will be no wider than the full image, and we divide it by two to
sample points evenly spread through the image. But for a subset frame,
we need to use a different sampling rate to ensure that the sampled
points are within the width of the frame.

Change-Id: I4a313db096fbaea7d869927a9da5df9beb9f6706
Reviewed-on: https://skia-review.googlesource.com/c/165500
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>

* Add SkColor4f for SkPixmap
* Add the SkShader variants