Make player hud tuner slightly more usable

[yohjimane]

• Delta pos / delta rot can no longer flip signs, instead they are incremented by a percentage of the current value. This allows for more fine-tune adjustments while making it easier to control the deltas.
• SDL_SCANCODE_KP_MULTIPLY doesn't exist on all keyboards, so switched it to SDL_SCANCODE_PERIOD to make level input redirection easier to use. This is so we can move the camera around while using the hud tuner, by using the demo_record functionality.