Improve the sink template caps for all encode.

The current static string caps of encode includes all supported formats and has too many formats, which causes a bad user experience, user can not know which format is useful. We have issue: #244 (closed)

We can improve this by query all supported chroma type of each profile and entrypoint at init time, and exclude RGB kind formats if not needed(So far only jpeg needs RGB format).

Fixes: #244 (closed)