Fuzz style customization with user shader

[Ludicrous_peridot]

Please allow fuzz style to be customized by modder via a user shader they provide with the mod.

One naive way this could work for a single global override is a new GLDEFS definition e.g. FUZZTYPE that would allow referencing a shader.
This type of shader would always register as 0 in usershaders.
In options menu a new value could be added to FuzzStyle, e.g. User Style that would reference the FIRST_USER_SHADER - SHADER_NoTexture.
GZDoom could ship with reference implementation of such user shader in its assets pk3 registered by default, that modders could base their customizations on.