app: avoid id. matrix short-circuit in trans. tool when ...

... the matrix is not actually used

Don't abort transform-tool commits when the transformation matrix
is the identity, for transform tools that don't calculate a
transformation matrix to begin with (i.e., the flip tool),
otherwise they do nothing.

app/tools/gimptransformtool.c

@@ -991,7 +991,8 @@ gimp_transform_tool_commit (GimpTransformTool *tr_tool)
   if (tr_tool->gui)
     gimp_tool_gui_hide (tr_tool->gui);
 
-  if (gimp_matrix3_is_identity (&tr_tool->transform))
+  if (GIMP_TRANSFORM_TOOL_GET_CLASS (tr_tool)->recalc_matrix &&
+      gimp_matrix3_is_identity (&tr_tool->transform))
     {
       /* No need to commit an identity transformation! */
       return;