stream encoding mode for desktop and monitor modes

[totaam]

Two different ways of implementing this:

• re-using the GStreamer CaptureAndEncode (gstreamer shadow capture #3750) to bypass most of the xpra rate, speed and quality controls and simply push video frames as packets:
  

  xpra/xpra/codecs/gstreamer/capture.py

  Lines 124 to 128 in 7c6dfc3

  	class CaptureAndEncode(Capture):
  	"""
  	Uses a GStreamer pipeline to capture the screen
  	and encode it to a video stream
  	"""

• re-using our own WindowVideoSource
  This one has the benefit of already handling all the bandwidth controls, including bandwidth limit, automatic downscaling, etc.

[totaam]

The commit above works reasonably well in most cases, but:

• with the gstreamer encoders, we end up making an extra memory copy of the xshm data: gstreamer zero copy #3754
• we could do something smarter when the number and size of the damage regions updated is small: just paste them over the previous image wrapper of the window - is it worth the effort and complications?

Using the CaptureAndEncode approach would save one memory copy.

[totaam]

With some preparatory work already done in 9a1ed42, 057e240 adds a pure GStreamer encoder that injects packets directly using the same code as the gstreamer shadow capture (#3750)

This mode is hidden for now because it isn't ready:

• the client must use --encoding=stream - just like above
• the server must be started with XPRA_STREAM_MODE=gstreamer

---

Many more changes are needed for this new mode:

• handle the window being resized - not working yet - not sure why, too many events? should be throttled anyway
• figure out why the damage extension is not working for the ximagesrc gstreamer element - perhaps add a gate that we control so we can decide when to start processing the next frame? similar to a leaky queue? ideally, blocking ximagesrc from producing more frames if we're not going to use them
• better sanity checks, the window may be resized and could become smaller than what the encoder can deal with - though monitor and desktop mode should already have a sensible minimum size
• if this is enabled for seamless mode, it should only be used for video windows, or at least not for text ones, not for popups, etc..
• make it easier to customise the pipeline used so we can use GPU encoders
• make it easier to enable this mode?

[totaam]

Now automatically enabling encoding=stream: 509af1a
And also automatically using the gstreamer encode if this gives us GPU acceleration.