Fix coordinate transform for GtkGestureStylus backlog
See #3491 (closed) and the commit messages.
I moved computation of the transform matrix to a separate function in gtkwidget.c such that both translate_event_coordinates()
and gtk_gesture_stylus_get_backlog()
can use the same code. In the process, I did refactor gtk_gesture_stylus_get_backlog()
a bit, please check that the argument for moving things out of the loop is actually correct.