--gamma: Say 'srgb' for sRGB.
gifview: Improve cache behavior.

Add a `--memory-limit` parameter, which defaults to 40MB. Cache
at most `--memory-limit` bytes of animation data on the X server.
gifview nit: Don't put frame numbers on title when animating.
Whoopsy
Add ordered dithering modes.

Ordered dithering avoids many animation artifacts. Use an algorithm
inspired by Joel Ylilouma's descriptions at
bisqwit.iki.fi/story/howto/dither/jy/. Add a new style of ordered
matrix, too: an ordered 64x64 matrix that is constructed randomly,
subject to local constraints that discourage artifacts.

Signed-off-by: Eddie Kohler <ekohler@gmail.com>