Pushing the buffer via gst_pad_push () in transform_ip () function causes downstream elements to process the buffer with a reference count > 1. This leads to performance issue if there are downstream elements which modify the buffer memory. However, in drop-only mode this reference is not required. So, let GstBaseTransform push the buffer in drop-only mode. Fixes #4258 Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9532>