Add quad overdraw mode. Thanks Stephen Hill (@self_shadow)!

* http://blog.selfshadow.com/2012/11/12/counting-quads/
* Quad Overdraw based on ScenePS4 from the revised implementation. The
  colours are new, up to 20 levels. Picking pixels shows the overdraw level
* Available per-pass and per-drawcall, a pass defined the same way as in
  the mesh view, drawcalls since last clear or RT change.
* List of events now passed through to RenderOverlay the same as RenderMesh

renderdocui/Windows/TextureViewer.Designer.cs

@@ -554,7 +554,9 @@
             "Stencil Test",
             "Viewport/Scissor Region",
             "NaN/INF/-ve Display",
-            "Clipping"});
+            "Clipping",
+            "Quad Overdraw (Pass)",
+            "Quad Overdraw (Draw)"});
             this.overlay.Name = "overlay";
             this.overlay.Size = new System.Drawing.Size(121, 25);
             this.overlay.SelectedIndexChanged += new System.EventHandler(this.overlay_SelectedIndexChanged);