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.
The use of this method was removed in:
commit 539f10f4d9
basecamerasrc: More cleanup
The code from wrappercamerabinsrc is from v4l2camerasrc but is unused:
get_allowed_input_caps is not called anywhere.
This is not implemented in any of our real sources to which wrappercamerabinsrc
might connect but this is optional and can be implemented at any time. A
limit on the software zoom level using video{crop,scale} would be arbitrary.
Adds a virtual function to basecamerasrc in case subclasses want to be
notified of changing preview caps. This is useful if the subclass wants
to post the preview itself or if it wants to provide a preview buffer
as close to as possible to the user's requested resolution to the
preview generation pipeline.
