Refactor use of math and other headers across the codebase.

* Delete mathstuff.h and defs.h.
* Move typedefs into util/types.h.
* Move definitions into util/defs.h.
* Use std::min and std::max for math_min and math_max.
* Replace math_clamp with a template function to prevent repeated sub-expressions.
* Add a Result enum for SoundSource::open() and SoundSource::parseHeader().
* Misc. other header-related cleanups.