If disposal method is set to BACKGROUND one would expect that
canvas should be restored to global background color. That is
what gif standard suggests. Most (all?) decoders however treat
this as reseting canvas to transparency or fixed, decoder
specific background color. Virtually all gifs are prepared with
this in mind so to not break them we can't follow standard here.
Change-Id: I90ca712bba89d4190771eb5320eabda353d3e2bb
This adds ability to view gif images in rockbox.
Works both on color and gray/monochrome targets (greylib).
Aspect correction is supported as well.
Limitations:
- animated gifs are restricted to 32 frames
- animated gifs loop always (loopcount is ignored)
- plain text extension is not supported
- animated gifs with interframe delay = 0 are treated as still
images (web browsers usually treat delay 0 as 100ms to prevent
exhaustive CPU load by such images)
Change-Id: I61501f801ddcd403410e38d83e6bddc9883e7ede
