This change removes convoluted core thread extraction through dispatch
added in 500ce69a06.
Currently we fully control this thread, create it and join, there is no
reason not to keep the handle of it in the player context.
As a bonus to code simplification this also fixes thread handle leak on
Windows.
Fixes: #14472
timeBeginPeriod() only allows setting minimum timer resolution
to 1 ms. However, modern x86 platforms support a minimum timer
resolution of 0.5 ms. Use NtSetTimerResolution() instead for
the increased resolution, which can be set with MPV_HRT_RES.
Additionally, change the units of mp_start_hires_timers(),
mp_end_hires_timer(), MPV_HRT_RES, and MPV_HRT_MAX to nanoseconds,
in accordance with other functions used in timer.h.