Implement previewing functionality using 2 properties. A boolean
(post-previews) that indicates if previews should be posted, and a
GstCaps (preview-caps) to provide the desired preview caps.
wrappercamerabinsrc implements previewing by supplying the captured
image to a pipeline to adapt it to the required caps before posting.
Instead of linking 3 src pads from tee to the ghostpads, use 2
srcpads and add an output-selector to completely split caps
negotiation of video/image modes. I don't think there is an
use case that would require image and video pads to be used
at the same time.
