Simplify rowsDecoded returned by incrementalDecode

Simplify rowsDecoded returned by incrementalDecode

When sampling, we previously had to figure out the number of rows that
were written to the output based on the value reported by
incrementalDecode. (We also messed up the first time - see the fix in
crrev.com/2343153003.)

Instead, make incrementalDecode report the actual number of rows
written to. This can be provided directly to fill.

Make SkPngCodec report the correct number of rows, by incrementing its
count when it actually writes to the destination.

This also will simplify my in progress GIF change.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2407543002

Review-Url: https://codereview.chromium.org/2407543002