Blender Git Loki

Implement a new dedicated weight painting shader.

Move the weight paint drawing to the fragment shader. The shader
uses a texture that uses the U.coba_weight custom color band, or
an internal color band.

In addition to actual weights, the shader has to display two
alert colors: missing vertex group, and zero weight. The zero
weight alert has to be blended with regular weight colors,
so that a single alert vertex surrounded by weighted ones is
still visible.

Reviewers: campbellbarton, fclem

Differential Revision: https://developer.blender.org/D3675