Fix an assert statement in SkPngCodec

allRowsCallback was asserting that
    rowNum - fFirstRow == fLinesDecoded

But it was expecting fFirstRow to be 0 (logically, it is, since we are
decoding all rows), and it never set it (because it doesn't really need
to, except for this assert). This is only a problem if we reuse an
SkPngCodec after previously scaling with it. Add a test that triggers
the assert, and fix it.
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Review-Url: https://codereview.chromium.org/2401133002
2 files changed