Implement perspective cloning from a pattern. This needed quite some
refactoring, so GimpSourceCore would be able call its get_source()
vfunc also for a GimpClone subclass: essentially this commit adds a
new GimpSourceCore::use_source() vfunc that replaces the
source_options->use_source flag hack, and makes sure the graph used in
GimpPerspectiveClone's get_source() impl reads from the pattern
instead the drawable.

This version does not properly tile the pattern yet, so you can only
clone one copy of the pattern (gegl:tile is not quite doing the right
thing, so it's disabled).