Add Quad Overdraw debug view mode

Source/Shaders/Editor/QuadOverdraw.shader

@@ -0,0 +1,34 @@
+// Copyright (c) 2012-2021 Wojciech Figat. All rights reserved.
+
+#include "./Flax/Common.hlsl"
+
+// [Quad Overdraw implementation based on https://blog.selfshadow.com/2012/11/12/counting-quads/ by Stephen Hill]
+
+Texture2D<uint> overdrawSRV : register(t0);
+
+float4 ToColour(uint v)
+{
+    const uint nbColours = 10;
+    const float4 colours[nbColours] =
+    {
+       float4(0,     0,   0, 255),
+       float4(2,    147, 25, 255),
+       float4(0,   255, 149, 255),
+       float4(0,   255, 253, 255),
+       float4(142, 250,   0, 255),
+       float4(225, 251,   0, 255),
+       float4(225, 147,   0, 255),
+       float4(225,  38,   0, 255),
+       float4(148,  17,   0, 255),
+       float4(255, 255, 255, 255)
+    };
+    return colours[min(v, nbColours - 1)] / 255.0;
+}
+
+META_PS(true, FEATURE_LEVEL_ES2)
+float4 PS(float4 svPos : SV_POSITION) : SV_Target
+{
+    uint2 quad = svPos.xy * 0.5;
+    uint overdraw = overdrawSRV[quad];
+    return ToColour(overdraw);
+}