toon color gradients

[ScanMountGoat]

Materials that render with the toon deferred shader use color ramps in monolib/shader. This can probably be represented as a color ramp in the material with values based on the selected shader database. Users are unlikely to edit the toon ramp textures. It may also be better to load the full texture from png and select a row with material nodes.

• [x] import gradients
• [x] export selected gradient row

[ScanMountGoat]

Editing the material parameter that selects the color gradient will require changes in xc3_model for https://github.com/ScanMountGoat/xc3_lib/issues/15.

[ScanMountGoat]

This should be its own node group to incorporate specular color. The toon deferred shader is completely separate in game from the pbr shader.

[ScanMountGoat]

Material edits for the toon V coordinate should use a node setup that's unlikely to change instead of relying on node naming.