SkWuffsCodec: swizzle only the dirty rect

This should result in a little less work, since the dirty rect should be
a subset of the frame rect. More importantly, the dirty rect will be
empty until we've fully read the color table. For incomplete data, we
therefore shouldn't read an uninitialized color table.

This also lets us convert from the Wuffs palette to the Skia palette
(fColorTable) once (per frame), not once per onIncrementalDecode call.
Again, this means we do a little less work.

Bug: oss-fuzz:11780
Change-Id: I1991778ca19f0525a0fefa57448e2dd015ea38fb
Reviewed-on: https://skia-review.googlesource.com/c/175900
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
2 files changed