Avoid awkward kludges with IMB_init/exit & IMB_ffmpeg_init calls.
The animation player was responsible for calling IMB_exit, not IMB_init,
this complicated drag & drop which needed to re-internalize IMB before
returning.
Delay initializing IMB since none of the other argument handlers
depend on it, make wm_main_playanim_intern responsible for calling
both initialization & exit calls for IMB.
Also expand code comments for ARG_PASS_SETTINGS.
704985152e