Evaluate non-palette compression for palette image

Evaluate if for Palette images (num_colors <= 256), non-palette
compression path (Subtract green, predictor transform etc) yield an
optimal compression density.

This change reduces the WebP file (for palette images) size by 0.4% with
drop of 3-5% in compression speed.

Change-Id: I1ad66fa94db4fd7ba7bc215763791ef662cd4f42