fix gradient pattern origin translation for time axis view items

note: the solution is non-obvious, but the obvious one of applying a translational Cairo::Matrix to the gradient
caused an exception to be thrown because of a non-invertible matrix.

libs/canvas/fill.cc

@@ -60,7 +60,14 @@ void
 Fill::setup_fill_context (Cairo::RefPtr<Cairo::Context> context) const
 {
 	if (_gradient) {
+		Cairo::Matrix m;
+
+		Duple origin = item_to_window (Duple (0, 0));
+
+		context->translate (origin.x, origin.y);
 		context->set_source (_gradient);
+		context->translate (-origin.x, -origin.y);
+
 	} else {
 		set_source_rgba (context, _fill_color);
 	}