Doxygen workarounds.
Math: removed superfluous instantiation sources.

Moved all instantiation into instantiation.cpp and removed the rest
(~200 lines of mainly license headers). Saves another ~3 seconds (4:06
before, 4:03 now). I'm done with these micro-optimizations for now.
De-inline TextureState constructor/destructor.

It is heavier than it looks. Also fixed some includes in Implementation
subdir.
Moved all global state into Renderer class, created implementation file.

Also updated and crosslinked documentation.