GIF: Better check for frame dependency

GIF: Better check for frame dependency

If a frame does not have a valid transparent index and it covers the
prior frame, it does not really depend on that frame. Instead, it
depends on the frame that the prior frame depends on.

Determine this once we have parsed the local color map (if any), so a
transparent index out of range of the color map is not considered
valid.

Share code that determines whether a frame has a transparent pixel.

Add a test that we compute the dependencies correctly. randPixelsAnim.gif
has 13 frames. After the first, the frames cover all combinations of

- Whether the prior frame was keep, restoreBG or restoreToPrevious
- Whether the new frame covers the prior frame
- Whether the new frame has a transparent pixel

(It only does so when using a global color table. It may make sense to
expand the test to also cover using local color tables.)

The test caught a bug where we incorrectly reused an existing
SkColorTable for a different frame. Fix that bug by keeping track of
the transparent index associated with the current SkColorTable.

Change-Id: I3cf6be7f612990fa7a00d9e74d116d31bd227526
Reviewed-on: https://skia-review.googlesource.com/6402
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>