Adding a layer difference may mean the pipeline overrides all of the
layers of its parent which might make the parent redundant so we
should try to prune the hierarchy.
This is particularly important for CoglGst because whenever a new
frame is ready it tries to make a copy of the pipeline it last used
and then replace all of the textures in the layers. Without this patch
the new pipeline would keep the parent pipeline alive which means also
keeping the old textures alive so all of the frames of the video would
effectively be leaked.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 576c7b55aa835448c977f1d79d128dffd40e7cd8)
22363a4137