(posix) don't create a new fobject when there's a vid in the feedslot already (avfeed/recordtarget bug)
(core) readback should check current state of feedfunc before passing on
(build/darwin) libretro3d build updated to reflect headless changes
(shmif) for SHMIF_SIGVID | SHMIF_SIGAUD, only actually trigger SIGAUD if there's audio buffered.