Regression fix for lossless decoding
Regression fix for lossless decoding

Reported here: https://code.google.com/p/webp/issues/detail?id=239

At the beginning of method 'DecodeImageData', pixels up to
'dec->last_pixel_' are assumed to be already cached. So, at the end of
previous call to that method also, that assumption should hold true.

Hence, we should cache all pixels up to 'src' regardless of 'src_last'.

This affects lossless incremental decoding only, as that is when
src_last and src_end differ.
Note: alpha decoding is implicitly incremental, as alpha decoding of
only the rows 'y_end - y_start' happens during FinishRow() call. So, this bug
affects alpha decoding in non-incremental decoding flow as well.

This bug was introduced in: https://gerrit.chromium.org/gerrit/#/c/59716.

Change-Id: Ide6edfeb2609b02aff701e1bd9fd776da0a16be0

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