Not possible to change bitrate and other encoder configuration without creating a new encoder context

[sdroege]

Describe the bug

Encoder configuration is currently only possible to provide when creating an encoder context. It would be useful to be able to change at least the bitrate (and quantizer?) while the encoder is running, while the encoder is running.

To Reproduce

In e.g. WebRTC-style scenarios where the bandwidth is limited and changing over time (as observed via different metrics and algorithms), it would be beneficial if the target bitrate could be changed while the encoder is running to allow adapting to this.

Expected behavior

Some API would be available to change selected encoder configuration while the encoder is already created and in use. The aom encoder supports this, for example.