Fix GIF image decoding: do not zero transparent pixels

Fix GIF image decoding: do not zero transparent pixels

For the special transparent color index, the decoder would skip
writing anything out (thus leaving the pixels at 0 rgba value).
Although correct for later frames, for the initial frame this would
loose the color information for such pixels (which one otherwise
could have made visible e.g. by converting then image to an
alpha-less image format).

Change-Id: I316cefce8f21797feedebfbf98296ad84eaa4b99
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
(cherry picked from qt/c309d424f45dc0e7b62fbbbabf20dbfe355f48a7)
Reviewed-by: aavit <eirik.aavitsland@digia.com>