va: encoders: Generate dynamically the rate-control property
The available rate control depends on the profile, but nowadays all the rate controls are exposed in the property, leading to frustration to users.
Ideally the rate-control should be generated in run-time. A difficulty is that an encoder element, before downstream negotiation, supports several profiles.
A proposal is to add a helper function in GstVaFilter pasing the codec and getting back the GParamSpec with all the rate controls supported by all the profiles available for that codec.