Remove IPO in favor of manual selection of small inlinable functions

[madmann91]

A couple of additional notes:

• The performance is similar to the renderer with IPO enabled, maybe a tiny bit slower.
• Compilation times could be improved by removing the use of a global include file (includes.h), and only including what is needed, not more.
• Some functions duplicate functionality. See add, muls, and lerp in color.c in the version of the renderer before this commit. I have removed the first two, but kept the last as it is used here and there. Its uses should probably be replaced by mixColors (or the other way around).

[madmann91]

It might make sense to re-enable IPO, though, but only in Release builds.