Issue #1925 - Rewrite NS_FOR_CSS_CORNERS to be similar to *_SIDES

- Renames it to NS_CSS_FULL_CORNERS and uses the one macro for all full
  corners. In preparation for giving half corners the same treatment.
- Swaps to prefix operator++

gfx/thebes/gfxRect.h

@@ -51,9 +51,6 @@ struct gfxRect :
             case NS_CORNER_TOP_RIGHT: return TopRight();
             case NS_CORNER_BOTTOM_RIGHT: return BottomRight();
             case NS_CORNER_BOTTOM_LEFT: return BottomLeft();
-            default:
-                NS_ERROR("Invalid corner!");
-                break;
         }
         return gfxPoint(0.0, 0.0);
     }