Reformat Mathlib.h to match Quetoo Vector.h style

- Doxygen @return/@brief on every function and union member
- Spaces around all binary operators (*, +, -, /)
- All one-liners expanded to proper K&R multi-line bodies
- Union members annotated with @brief (Array accessor, Component
  accessors, Swizzle)
- vec3/vec4/mat4 briefs corrected from HLSL to GLSL
- M_PI #define moved to column 0 (was indented with leading spaces)
- vec3_cross expanded to multi-line for readability
- mat4_rotation, mat4_mul, mat4_inverse: spaces around all * operators
- mat4 @details flat accessor comment uses spaces around operators

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jdolan