Draft: Screensharing

[nlevnaut]

WIP PR to implement #398

Would expect this to ultimately cover:

• Interaction with screensharing object:
  • Start screensharing
  • Stop screensharing
  • One player controls a screensharing object at a time
  • Release screensharing object upon controlling-user disconnection
• Start/stop usage of display media track via hydrogen
• Render current screensharer's display stream to VideoTexture and apply texture to screensharing object
• Provide a way to create a screensharing object in a world

Originally I was approaching this implementation similarly to how portals are done, but portals are based on the OMI_link extension. It may make more sense for the screensharing object to be a UI element, unless perhaps something clever can be done using EXT_texture_video.