Fix swapped pixels in GifDecoder's output

[mtopolnik]

StandardGifDecoder uses modified public domain code to decode the GIF. Some changes were done to it that introduced a subtle bug: the initial two pixels are swapped, as well as the pixels after each CLEAR command in the LZW stream.

Comparing with the original code, the bug was introduced because the loop that pops the bytes from the stack into the output pixel array was moved. This code motion did not take into account the continue in the branch guarded by oldCode == NULL_CODE. The pixel that this branch pushes to the stack was supposed to be immediately transferred to the output, but since the stack-popping loop was moved, it happens after adding first to the output pixels, thereby swapping the two initial pixels.

The fix is simple: instead of pushing to the stack in the oldCode == NULL_CODE branch, send the pixel directly to the output pixel array.

This PR includes a fix, a small test GIF with 8 white-black-white-black pixels, and a JUnit test that fails without the fix.

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[mtopolnik]

I signed the CLA just now.

[googlebot]

CLAs look good, thanks!

[stale]

This issue has been automatically marked as stale because it has not had activity in the last seven days. It will be closed if no further activity occurs within the next seven days. Thank you for your contributions.

[mtopolnik]

Thank you for your attention, googlebot and stalebot.

[sjudd]

Thank you! Sorry for the delay. Nice find.