OSX: Raw input without acceleration (raw USB HID parsing)

Spawns a separate thread that parses only mouse x/y changes while the
rest of input is handled by SDL2, like buttons/keyboard etc.

in_raw 0 uses SDL2 mouse x/y but the thread is still running in the
background, just that we don't use the values read.

Original source taken from fodquake, but pretty heavily modified.
OSX: Add support for K_CMD in keytable