Add overlays to clear before current draw/pass. Refs #164

* Depth is not cleared, only colour buffers.
* This uses the same definition of pass as the quad overdraw overlay,
  which isn't quite the same as the auto-grouping passes (which allow
  for varying colour binds being grouped together). It only counts up
  to the last draw where the outputs were different.

renderdocui/Windows/TextureViewer.Designer.cs

@@ -560,6 +560,8 @@
             "Viewport/Scissor Region",
             "NaN/INF/-ve Display",
             "Clipping",
+            "Clear Before Pass",
+            "Clear Before Draw",
             "Quad Overdraw (Pass)",
             "Quad Overdraw (Draw)"});
             this.overlay.Name = "overlay";