ShellRecorder: drop frames to approximate the target framerate

Instead of adding every rendered frame into the recording, drop frames
and only buffer and record enough frames to match the target framerate.

Increase the default frame rate from 15 to 30, since now that we're
actually enforcing framerate, it's noticeable that 15fps is not smooth.

https://bugzilla.gnome.org/show_bug.cgi?id=669066