Since we now make HTTP requests from addFilter, this makes
startup slightly faster.

This avoids throwing if the user changes their setting before
login.  Thanks to Jean-Jacques Sarton.

We used to initialise MediaPipe at worker start, which prevented
us from handling errors.  We now do it at filter init, and stop
the stream with an error message if initialisation fails.

We used to ignore errors in filter init, which could cause us
to send unblurred videos to the server.

We used blur the image then mask, which caused a halo around
the foreground.  We now mask then blur.

Thanks to Matohara Kenichiro.

This doesn't currently do anything useful, since we're running
tflite on the CPU, but might avoid bugs if we get GPU support.

This should free the underlying textures without waiting
for a GC to happen.

It doesn't work.

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.