[GStreamer] Track the size set on the caps

This is to avoid reconfiguring the decoder when setSize() is called
with the same size that is already in the caps. Originally, the sink
believes that its size is 0x0, so it always tries to reconfigure
the first time a size is set, even if it's the same as the already
configured size. This patch fixes that.
[GStreamer] Discover the natural video size as early as possible

This allows the video sink to have the correct size (respecting aspect ratio)
negotiated before it gets to preroll, which avoids software frame scaling
on the RPI, where scaling is done in the decoder.
[GStreamer] Set GST_PLAY_FLAG_NATIVE_VIDEO in playbin for the RPI platform to disable videoscale/videoconvert

The omx decoder should be able to provide exactly what the sink wants.
Software scaling/conversion is too slow.
[GStreamer] Set debug category in cairoimagesurfacebufferpool