We already support shaders on shape and zone entities. We also support material entities on shapes and zones. Combine these two features to allow people to apply shaders to models and avatars.

Overview:

- Material Entity JSON descriptions will support a new "hifi_simple_shader" model, with the same fields as the existing userData procedural fields.

- Procedurals on meshes will be disabled by default. Meshes with procedurals on them won't render at all. This setting can be toggled in the Graphics preference dialog and the Developer -> Render menu.

- Because the procedural data is a part of the entity properties, Entity Edit Filters can be used to whitelist/blacklist shaders.

This work is done here: https://github.com/highfidelity/hifi/pull/15577