Pass uniforms to custom shaders

[maniflames]

Thanks for all the hard work on this library! Right now I'm working on a small game and want to use custom shaders for post effects. Because of #58 and the documentation I got very basic one running (basically a reimplementation of the basic material). The only thing I'm struggling with right now is defining my own uniforms and passing them to the custom shaders.

Is there a way to do this with basic_pipeline and material::basic::Custom currently or would it be less of a hassle to fork three and modify renderer/mod.rs?

(I'm not entirely sure but this might be a duplicate of #174 if that is the case I'll close this issue and would love to chat over there 😄)

[kvark]

Hi! Custom pipelines where pretty much rolled in because I needed them for plane_splitter. So they only contain as much functionality atm as I needed, and you'd need to expand it (in three-rs code) if that's not enough.

FYI, the crate is in maintenance state until it's ported to a new graphics backend, see #222