convertframe: Use refcounting for the context

convertframe: Use refcounting for the context

While this creates a circular reference between the pipeline and the
context, this ensures that the context stays alive for as long as any
callbacks could be called on it. The circular reference is broken once
the conversion is finished (or error, or timeout), which will then cause
everything to be freed.

Previously it was possible that a callback could be called on the
context right after it was freed already.

Also use only a single context structure, the second structure does not
simplify anything and duplicates storage.