Introduce a custom coroutine/fiber scheduler to prioritize UI.

The viewer's main thread's main fiber is responsible for coordinating just
about everything. With the default round_robin fiber scheduling algorithm,
launching too many additional fibers could starve the main fiber, resulting in
visible lag.

This custom scheduler tracks when it switches to and from the main fiber, and
at each context switch, how long it's been since the last time the main fiber
ran. If that exceeds a certain timeslice, it jumps the main fiber to the head
of the queue and resumes that instead of any other ready fiber.

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