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.
This seems to work on gcc, clang and mingw as-is, but I made it
conditional on __GNUC__ just in case, even though I can't figure out
which compilers we care about that don't export this define.
Also replace all instances of assert(0) in the code by MP_UNREACHABLE(),
which is a strict improvement.
The existing thread pool code is the most primitive thread pool
possible. That's fine, but one annoying thing was that it used a static
number of threads. Make it dynamic, so we don't need to "waste" idle
threads.
This tries to add threads as needed. If threads are idle for some time,
destroy them again until a minimum number of threads is reached.
Also change the license to ISC.
