Remove the GimpSymmetry::get_operation() virtual function, and
instead implement gimp_symmetry_get_operation() by returning an
appropriate gegl:transform node based on the matrix returned by
gimp_symmetry_get_matrix().  The returned node is owned by the
caller; since we no longer use the node's identity for caching
trnasformed brushes, we no longer cache the transformation nodes.

Remove the function's paint_width and paint_height parameters, and
instead return a transformation that should be applied at the
center of the brush.  This simplifies the application of the
transformation in the Clone and Heal tools, as per the next commit.

Remove the implementation of GimpSymmetry::get_operation() from all
its subclasses, which should now only implement
GimpSymmetry::get_transform().