cssshadowvalue: Remove the move_to in paint_layout

_gtk_css_shadow_value_paint_layout is always called from
gtk_theming_engine_render_layout, which always moves the current point
on the context.

Since we should always have a current point before calling this
function, add an assert do validate that assumption.

gtk/gtkcssshadowvalue.c

@@ -393,8 +393,7 @@ _gtk_css_shadow_value_paint_layout (const GtkCssValue *shadow,
 {
   g_return_if_fail (shadow->class == &GTK_CSS_VALUE_SHADOW);
 
-  if (!cairo_has_current_point (cr))
-    cairo_move_to (cr, 0, 0);
+  g_assert (cairo_has_current_point (cr));
 
   cairo_save (cr);