Write transparent pixels more often in SkGifImageReader

Write transparent pixels more often in SkGifImageReader

This stems from a behavior difference between Skia and Chrome.
In Skia, we want to write transparent pixels as often as possible.
(It's faster than checking if we should skip each pixel.)
In Chrome, they avoid writing transparent pixels unless
absolutely necessary.

We were cautious about changing behavior when this first landed,
but this is easier to think about in a smaller change (right now).
(1) We can always write transparent pixels when we are writing
    an independent frame.
(2) There is no need for the progressiveDisplay() check.  We
    only ever use progressive display methods on the first
    frame - and the first frame is always independent.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4379

Change-Id: I82048a08e2003aac216f483c7db8df997b687149
Reviewed-on: https://skia-review.googlesource.com/4379
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>