Draft: Issue #12144: core, widgets: Improve filter performance

After 59b7b6a5, we had reports that filters ran much slower. This patch is an initial attempt to improve performance. It's a test for now, and more work likely needs to be done.

It reduces calls to gegl_node_get_bounding_box (), which may re-render the graph each time it's called. We should now only try to recalculate the crop when committing or resizing the filter.

Note that @ok said gegl_node_get_bounding_box () should only re-render if the properties of the node or ones affecting it are changed. We do have code in GimpDrawableFilter that makes those kinds of changes, so we'll see if this change improves performance.