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
3da46a8d8d