gegl_transform_is_composite_node() returns TRUE if the op's producer
is a (suitable) transform op.  OTOH,
gimp_transform_is_intermediate_node() returns TRUE only if *all* the
op's consumers are transform ops.  In other words, if transform op A
feeds to both transform op B and non-transform op C, A will not
consider itself an intermediate node, while B *will* consider itself
a composite node.  This results in double application of A's matrix:
first by A, and again by B.

Fix this by having gegl_transform_is_composite_node() check if the
source node is an intermediate transform node (i.e., if all its
consumers are transform ops.)