core: make WAKEUP_PERIOD overridable by the vo

This is better than having just the operating system type decide the
wakeup period, as e.g. when compiling for Win32/cygwin, a wakeup period
of 0.5 would work perfectly fine.

Instead, the default wakeup period is now only decided by availability
of a working select() system call (which is the case on cygwin but not
mingw and MSVC) AND a vo that can provide an event file descriptor or a
similar hack (vo_corevideo). vos that cannot do either need polling for
event handling and now can set the wakeup period to 0.02 in the vo code.

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