Add Quad Overdraw debug view mode

Source/Shaders/Editor/QuadOverdraw.hlsl

@@ -0,0 +1,48 @@
+// [Quad Overdraw implementation based on https://blog.selfshadow.com/2012/11/12/counting-quads/ by Stephen Hill]
+#ifndef __QUAD_OVERDRAW__
+#define __QUAD_OVERDRAW__
+
+RWTexture2D<uint> lockUAV : register(u0);
+RWTexture2D<uint> overdrawUAV  : register(u1);
+RWTexture2D<uint> liveCountUAV : register(u2);
+
+void DoQuadOverdraw(float4 svPos, uint primId)
+{
+	uint2 quad = svPos.xy * 0.5;
+	uint prevID;
+	uint unlockedID = 0xffffffff;
+	bool processed  = false;
+	int lockCount  = 0;
+	int pixelCount = 0;
+
+	for (int i = 0; i < 64; i++)
+	{
+		if (!processed)
+			InterlockedCompareExchange(lockUAV[quad], unlockedID, primId, prevID);
+		[branch]
+		if (prevID == unlockedID)
+		{
+			if (++lockCount == 4)
+			{
+				// Retrieve live pixel count (minus 1) in quad
+				InterlockedAnd(liveCountUAV[quad], 0, pixelCount);
+
+				// Unlock for other quads
+				InterlockedExchange(lockUAV[quad], unlockedID, prevID);
+			}
+			processed = true;
+		}
+		if (prevID == primId && !processed)
+		{
+			InterlockedAdd(liveCountUAV[quad], 1);
+			processed = true;
+		}
+	}
+
+	if (lockCount)
+	{
+		InterlockedAdd(overdrawUAV[quad], 1);
+	}
+}
+
+#endif