modularize shaders

[loganzartman]

the shaders are a mess with tons of copy-pasted code.

I already tried glslify, but it is broken for webgl2. there's already a PR open that might fix all my problems, but glslify seems pretty dead. It might be easiest to define shaders in JS with a glsl template literal, and use string interpolation to do some very basic code reuse.

[loganzartman]

wrote a 100 line shader preprocessor that does the job for now