Automatic stencil at export from VCol layer

Setting up stencil layers with the blend ramp by hand kinda sucks, and you need to make sure to get the flags right.

PlasmaMax has a feature to allow artists to use vertex colour layers to define the blending stencils. Somewhere back in the pyprp days, either Nadnerb or myself (or both?) wrote a script to turn a VCol layer into a UV mapping for a stencil, but it wasn't automatic and could easily get out of sync.

My proposal is that we add a field to the per-layer Plasma panel with a checkbox for "Use vertex colour blending mask" and a select box for which VCol layer to apply as a stencil. At export time, this would automatically create the blending layer with the right texture, right flags, and UV coordinates.
For simplicity, we'd handle the VCol colours the way we do for alpha: a range from white to black.

The issue names no files, tests, or entry points. Trace the per-layer Plasma panel and export path first, then define how the selected vertex-colour layer produces the blending stencil, texture, flags, and UV coordinates. Done means the option works automatically and treats vertex colours as a white-to-black alpha range.