libobs: Defer media controls to video thread

To prevent sources from having to take too much extra consideration into
threading, defer the media control functions that directly affect
functionality to the video thread. Getters will still have to take
threading into account, but this should make things much more trivial
for media controls thread-wise.

(Lain note: Context: I noticed that things such as the slide show
require mutexes due to their media controls, and I felt that it was
largely unnecessary and that libobs should mitigate the threading issue
itself and keep it all in the video thread like it should be. Again, the
getters are still going to require *some* consideration into threading
but in terms of threading, for the type of stuff we're doing, querying
state is far more trivial to take into consideration.)