vout: use a function to render the SPU

Extracted from !4088 (closed).

We can call it with different parameters depending on when/who is calling. We might also use it to render them for the snapshot, separately from the display, so it's always rendered for the display.

This will likely be needed as with SPUs in the black bar, we don't want a snapshot of the whole windows, just the video area but with SPU (and no OSD). Otherwise we don't need snapshots, the OSes have screen captures.