Optimize shaders for dithered gradients

It's faster to compute a dither calculation in the vertex shader and use
a varying (letting the GPU interpolate the fragment values) than to perform
that calculation in the fragment shader as part of a texture lookup.

Issue #7207600 Prime mr1 shader performance issues
Issue #7158326 Bad framerates on MR1 (Mako, Manta, Prime)

Change-Id: I15789582a6e9e2d8b9dd22aa5b0f72f0ba1cce7f