Currently we only support effects with N sinkpads and one single srcpad.
Apart from `gesaudiomixer` and `gescompositor` which can be used as effects and where sinkpads will be requested as needed based on the timeline topology GES will always request at most one sinkpad per effect (when required).
Note: GES always adds converters (`audioconvert ! audioresample ! audioconvert` for audio effects and `videoconvert` for video effects) to make it simpler for end users.