Add frame_delay parameter to output frame signals

First steps towards getting smart about this stuff :)

Particularly looking for:

• comments on the interface: objections to putting it in wlr_output, or problems raised by compositors not receiving this signal straight away? the old docs on wlr_output don't make guarantees about the timing of the signal, so in theory it's fine
• confirmation that the DRM backend change won't blow anything up

Tested via the touch example - I can reliably tell the difference between 0 and 12ms delay.