Eevee: Fix contact shadow creating to overshadowing

This was because the shadow rays could be generated below the geometric
normal.

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

@@ -226,6 +226,10 @@ float light_visibility(LightData ld, vec3 W,
 
 			vec3 ray_ori = viewPosition;
 
+			if (dot(viewNormal, ray_dir) <= 0.0) {
+				return vis;
+			}
+
 			float bias = 0.5; /* Constant Bias */
 			bias += 1.0 - abs(dot(viewNormal, ray_dir)); /* Angle dependent bias */
 			bias *= gl_FrontFacing ? data.sh_contact_offset : -data.sh_contact_offset;