Create preview pipeline already in initialization phase. This speeds
up NULL_TO_READY state change. Also implement a separate function for
setting the preview filter element.
This also restricts the preview filter property to work only on
NULL state.
Implements a state indicating flag to preview pipeline,
so that new caps are not set if the pipeline is processing a
preview. The caps are set as pending and applied when the
next preview post is called.
In this case a wait was implemented in the post_preview function,
so that new preview image buffer will wait until the other previews
have been posted to the application and the new caps can be used
safely.
