need to pass a GstSample to the utilitary preview buffer post functions
as a GstBuffer doesn't have caps anymore. The GstSample has the GstCaps
and it is used to inform the preview's pipeline about the format of the
input, before it gets converted to the user's requested output format.
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.
