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
This change essentially removes mp_thread_self() and instead add
mp_thread_id to track threads and have ability to query current thread
id during runtime.
This will be useful for upcoming win32 implementation, where accessing
thread handle is different than on pthreads. Greatly reduces complexity.
Otherweis locked map of tid <-> handle is required which is completely
unnecessary for all mpv use-cases.
Note that this is the mp_thread_id, not to confuse with system tid. For
example on threads-posix implementation it is simply pthread_t.
