* app/text/gimptextlayout-render.c: remove private function which
creates the transform matrix.

* app/text/gimptextlayout.[ch]: add it here as public API. Also add
function which transform and untransform PangoRectangles, points and
distances using the transform matrix.

* app/tools/gimptexttool.c: convert coordinates using above new
transform functions when drawing selection and cursor, and when
processing mouse events.