Add is-live=false mode (Blocking v buffer creation on OnPaint)

[aiden-jeffrey]

We block create on a fresh OnPaint call - effectively frame locking the cefsrc output to 1 frame per requestAnimationFrame on the browser side. This can be tested with http://localhost:9000/raf_test.html?audioDelay=-1. For example:

gst-launch-1.0 -e cefsrc is-live=false url="http://localhost:9000/raf_test.html?audioDelay=-1" ! video/x-raw, width=1920, height=1080, framerate=60/1 ! cefdemux name=demux ! queue ! videoconvert ! queue max-size-bytes=0 max-size-buffers=0 max-size-time=3000000000 ! x264enc ! queue ! mp4mux name=muxer ! filesink location='test.mp4' audiotestsrc do-timestamp=true is-live=true  volume=0.0 ! audiomixer name=mix ! queue max-size-bytes=0 max-size-buffers=0 max-size-time=3000000000 ! audioconvert ! audiorate ! audioresample ! avenc_aac bitrate=128000 ! queue ! muxer. demux. ! queue ! mix.

NB: setting audioDelay=-1 in that test page turns off audio. This change doesn't play well with audio currently.

Note that as @MathieuDuponchelle suggested, this clogs up the audio side of things - I need to dig further into my audio PR to genuinely decouple the audio buffer creation from the video buffer creation.

Based on #104. So the actual diff is this commit.

There's also this commit - apparently we should return NO_PREROLL on state change to PAUSED https://gstreamer.freedesktop.org/documentation/additional/design/live-source.html?gi-language=c, but if we do this the pipeline never goes to playing.