Change the logic adjusting the Histogram bits.

Updated the logic to limit the Histogram size to a constant, instead of
computing the same based on the Histogram size (that's variable size based on
the cache bits) for the maximum possible cache bits. The actual cache bits may
be lower than the maximum.
Note: The constant 2600 is 16MB/Sizeof(HistogramSize(MAX_COLOR_CACHE_BITS)).

The compression density remains the same with this change, with little faster
compression speed.

Change-Id: I3149894962852e9dad2501b9aa16bb847a20fd86
Increase the MAX_COLOR_CACHE_BITS from 9 to 10.

The Maximum allowed limit is 11.
The Q=25 and below is not impacted as cache bits are forced to 0.
This saves 0.05% - 0.1% bytes for other quality with almost same compression
speed (+/- 2-3%, that's more of a noise).

Change-Id: Icf972a98f298c89e140e37a627baf709134be9a0
gif2webp: Use the default hint instead of WEBP_HINT_GRAPH.

This is much faster and the compression is slightly better too.

Change-Id: Ibf0d10eea83bfabfcc44ee497074767462ff41b1