Check if vblank waits were released before waking.
Add vblank wait callback handlers.

Turns out it's a deadline to wake up by, which makes sense.

sceDisplayWaitVblankStartMultiCB 10
   -> Callback
      sceDisplayWaitVblankStartMultiCB 2
      +2 vblanks
   <- Thread
+8 more vblanks