SkWuffsCodec tells wuffs to decode only the current frame's
contribution, and then SkWuffsCodec blends it onto the prior frame (if
any). If there is a prior frame, do not call SkSampler::Fill, and blend
the new frame onto the old, rather than replacing it.
In addition, set rowsDecoded to scaledHeight to work properly for
scaled decodes.
TBR=reed@google.com No change to public API
Bug: skia:8235
Change-Id: I08e89f1083da3f9b98f93d8d2375ce78f0c378f8
Reviewed-on: https://skia-review.googlesource.com/c/171645
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Nigel Tao <nigeltao@google.com>
9b0ba2c2cd