ClutAnimDemo: Fix it for gpu's with other than 256-slot gamma tables.

The demo made the - historically correct, but nowadays sometimes wrong
- assumption that the gpu gamma tables always have 256 slots. Not true
on, e.g., OSX 10.9 + Nvidia gpu’s, not true under Linux with many
modern Nvidia gpu’s.
Fix this, and clean up the demo and its help text a little bit along
the way.

(cherry picked from commit 1705932520936d55e4048ef34896c87d5b3fe1ce)