EEVEE: RenderPass: Improve AO pass if screen space radius is small

This just bypass the occlusion computation if there is no occlusion
data. This avoids weird looking occlusion due to the screen space
geometric normal reconstruction.

source/blender/draw/engines/eevee/shaders/effect_gtao_frag.glsl

@@ -99,9 +99,12 @@ void main()
 
     OcclusionData data = occlusion_load(vP, 1.0);
 
-    float visibility = diffuse_occlusion(data, V, N, Ng);
-
-    FragColor = vec4(visibility);
+    if (min_v4(abs(data.horizons)) != M_PI) {
+      FragColor = vec4(diffuse_occlusion(data, V, N, Ng));
+    }
+    else {
+      FragColor = vec4(1.0);
+    }
   }
 }