Shader includes and variants

[adriengivry]

Problem this feature should fix
Having to deal with only one file per shader is sometimes a problem (Code duplication).
For instance, it would be nice to share our vertex or our fragment shader between all shaders.
It would also be nice to have a shader library (BRDF, tools, noise...) available for other shaders.

Expected solution
We should implement shader include and variant systems (#define, #ifdef).

[adriengivry]

Maybe this is the right time to introduce our own shader syntax. We can inspire from ShaderLab, which appears to be very convenient.

[adriengivry]

Duplicate of #104