themingengine: Don't draw 0px borders

Theming code gets confused when computing the spacing for 0px wide dots
and then divides by 0. And then cairo complains and stops drawing
anything forever out of spite and then we end up with a single color
screen.

https://bugzilla.gnome.org/show_bug.cgi?id=721800

gtk/gtkthemingengine.c

@@ -1591,6 +1591,13 @@ render_border (cairo_t       *cr,
       if (hidden_side & (1 << i))
         continue;
 
+      /* NB: code below divides by this value */
+      /* a border smaller than this will not noticably modify
+       * pixels on screen, and since we don't compare with 0,
+       * we'll use this value */
+      if (border_width[i] < 1.0 / 1024)
+        continue;
+
       switch (border_style[i])
         {
         case GTK_BORDER_STYLE_NONE: