Add shaders

Sources/Shaders/SnowedSpeedTreeBillboard.cginc

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+	
+	uniform float4    _GS_SnowData1;	// x = relief, y = occlusion, z = glitter, w = brightness
+	uniform float4    _GS_SnowData2;	// x = minimum altitude for vegetation
+	uniform float4    _GS_SnowData4;    // w = billboard coverage
+
+	
+	// get snow coverage on tree billboards
+	void SetTreeBillboardCoverage(Input IN, inout SurfaceOutput o) { 
+		
+		float minAltitude = saturate((IN.worldPos.y - _GS_SnowData2.x) * 0.1);
+		float solid       = o.Albedo.g * 2.0 - _GS_SnowData4.w;
+		float snowCover   = minAltitude * saturate(solid) * 0.95;
+
+		// pass color data to output shader
+		o.Albedo = (1.0 + o.Albedo.g) * (0.25 + _GS_SnowData1.w) * solid;
+		o.Alpha  = snowCover;
+	}