compositor: doesn't convert from RGBA to I420

When I have compositor element that combines RGBA and I420 streams and it always produces RGBA stream at the output. Forcing output to I420 with caps causes issues with RGBA source ("streaming stopped, reason not-negotiated (-4)"), since that source was tried to be forced to I420, which it can't do because of alpha channel.

While documentation says:

Compositor will do colorspace conversion

It feels like adding videoconvert element in this case is an overhead that can be avoided if compositor was smart enough to produce correct format right away.