Modular shaders

[cochrane]

Currently there is a lot of code duplication between all shaders. That is annoying from a maintenance point of view and must be reduced.

Instead, implement a modular system where there is one template shader, and the different parts of the template (e.g. "get diffuse color", "get normal" and so on) are instantiated in different ways as needed. The different parts of the template will probably be function calls that get implemented by various shaders; the resulting shader is generated by linking the right versions together.

This is a prerequisite for #56.

[cochrane]

Basically done but not yet used for OBJ

[cochrane]

Done along with the Metal port.