[rust png] Don't look for more `fcTL` chunks during incremental decode.

`SkPngRustCodec::onGetFrameCount` may consume the input stream to search
for additional frame metadata (i.e. searching for additional `fcTL`
chunks).  Moving stream position can confuse an incremental decode which
may be in progress.  Therefore after this CL `onGetFrameCount` will not
consume the input stream if there is an active decode in progress.  This
CL to some extent mimics how `SkWuffsCodec::onGetFrameCount` checks
`!incrementalDecodeIsInProgress`.

Bug: chromium:356922876
Change-Id: Icd0dea2f40cd2d0d15775fe256c287ada4b8e197
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/913917
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Ɓukasz Anforowicz <lukasza@google.com>
2 files changed