Drop a frame if a filter takes too long to run.

This avoids an unsightly warning when an ICE candidate arrives
late.

When a user sends a file to oneself (which is only possible
as a chat command), then the two file transfer data structures
have the same id, which causes confusion.  We used to detect
this case too late, detect it earlier.

Thanks to J.-J. Sarton for the report.

This avoids a collision with the close video button.
Thanks to J.-J. Sarton.

We used to copy the RTCPFeeback field from the up track.  It is
more correct to regenerate it with the exact feedback types
that we expect.

Remove support for WebGL contexts, use concrete types.

Document the new perimssion "caption".

It is required for displaying captions.