Avoid creating a Cairo buffer if all telemetry options are set to 0.

GStreamer's "theoradec" element always calls th_decode_ctl() for all telemetry
options, passing 0 if they are not enabled. This results in our creating a
Cairo buffer and performing YUV/RGB conversion, even if all telemetry is
disabled, which is very slow and can cause noticeable lag during playback.