Basically we know that if we are using mixing, compositor will be
able to do video conversion and scaling for us, so avoid adding those
usless elements.
This optimizes a lot caps negotiation for deeply nested timelines.
We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
With two exceptions:
* ges_clip_create_track_elements_func
* ges_uri_clip_set_uri
which were never declared in headers and should always have been static.
