Rename the last-buffer property to last-sample and make it return the new
GstSample type so that we can include caps and timing info in one nice bundle.
Add a new enable-last-buffer property. When false, it disables storing the last
received buffer in basesink::last-buffer. This can be useful in cases where
buffers need to be released asap.
API: GstBaseSink::enable-last-buffer