Gets a sample from the pipeline of the currently displayed image in preview, in the specified format.
Note that if you use "ANY" caps for caps, then the current format of the image is used. You can retrieve these caps from the
returned sample with get_caps.
| this |
A Pipeline in PLAYING or PAUSED |
| caps |
Some caps to specifying the desired format, or |
|
A sample of this's current image preview in the format given by |