Move the bulk of exec() into a new pump(). Since SDL wants to drive the
event loop itself, this is a requirement. We also add a WaitMode flag to
allow for immediately pumping events -- again, this is required because
SDL wants to be in full control of the event loop, and not let us wait.
The scheduler expects m_select_timeout to act as a deadline. That is, it
should contain the time that a task should wake at -- but we were
directly copying the time from userspace, which meant that it always
returned virtually immediately.
At the same time, fix CEventLoop to not rely on the broken select behavior
by subtracting the current time from the time of the nearest timer.
