less warped cluster distribution for 100% sky tile

less warped cluster distribution for 100% sky tile

Assets/ScriptableRenderPipeline/HDRenderPipeline/Lighting/TilePass/Resources/lightlistbuild-clustered.compute

 	GroupMemoryBarrierWithGroupSync();
 #endif
 	dpt_ma = asfloat(ldsZMax);
+	if(dpt_ma<=0.0) dpt_ma = VIEWPORT_SCALE_Z;		// assume sky pixel
 #endif
 
 	float2 vTileLL = float2(viTilLL.x/(float) iWidth, viTilLL.y/(float) iHeight);

Assets/ScriptableRenderPipeline/fptl/lightlistbuild-clustered.compute

 	GroupMemoryBarrierWithGroupSync();
 #endif
 	dpt_ma = asfloat(ldsZMax);
+	if(dpt_ma<=0.0) dpt_ma = VIEWPORT_SCALE_Z;		// assume sky pixel
 #endif
 
 	float2 vTileLL = float2(viTilLL.x/(float) iWidth, viTilLL.y/(float) iHeight);