Adds a persistent variable that keeps track of how much delay has been introduced between frames.
This variable is microsecond-precise, and gets decreased by 10 milliseconds per 10 milliseconds of measured delay (The minimum delay possible in GIF frames is 10ms).
During the process NEWTICRATE was switched out for the function I_GetTimeMicros.
This has the side effect of being able to support different TICRATEs and NEWTICRATERATIOs because it just counts the real time that passed between frames instead of relying on an internal framerate.
