Replace backslashes with forward slashes in include paths

Rationale: Forward slashes work as path delimiters on both windows and linux,
while backslashes only work on windows.
#include <cmath> where needed
Fix various file naming issues

- Make sure the letter cases in filenames match how they are referenced
  in the source code

  On Linux, file lookup is case-sensitive.
  e.g. "boss1hit.ogg" and "boss1Hit.ogg" are two different files.

- Fix 'settings.png' being referenced as 'settings.jpg' in the source code