Variables, props, and MFDs should round-robin their updates

[JonnyOThan]

Right now it's possible for the computer's variables, prop text, and all MFDs to redraw on the same frame. Each of these is individually timesliced but they don't have any knowledge of each other. A smarter system would make sure that no two of these systems updated on the same frame, to try to even out performance.

Might even want to timeslice MFDs across multiple frames, but I'm not sure if it would look weird if they don't all update at the same time.