Simulcast support

what is the intention regarding simulcast support?

We have the following options:

• Not support it, and make the app create N encoder instances, and make the app support it. Not very likely as currently there is no support for accessing the image raw data, so it will have draw the media stream in n different canvas, downscale it, create the capture stream for each one and pass each one to an instance of the decoder.

• Support it as configuration parameter of an encoder. This would allow to have single input for the simulcast encoder, but would make it much difficult to provide a good api compatible with non-encoders.

• Provide a helper/adapter that can be created with a webrtc encoding-like object parameter and will create internally n encoders, exposing each one for controlling each simulcast layer individually. This would allow to have a single input, and the simulcast adapter will internally downscale/forward it to each of the encoders individually.

I think that the latest one is the current approach in libwbertc internal code and also my preferred option.

Review issue #44 and the three simulcast approaches it outlines, including the libwbertc internal-code approach mentioned in the discussion. Before implementation, establish which API direction is intended; done would require a decided design rather than choosing among unresolved alternatives.