Use float textures to render gradients when possible

Float textures offer better precision for dithering.

In addition this change removes two uniforms from gradient shaders.
These uniforms were used to dither gradients but their value is
a build time constant. Instead we hardcode the value directly in
the shader source at compile time.

Change-Id: I05e9fd3eef93771843bbd91b453274452dfaefee