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
