Clean up deferred shaders by moving common methods into shared glsl file

indra/newview/app_settings/shaders/class1/deferred/diffuseAlphaMaskF.glsl

@@ -37,11 +37,7 @@ VARYING vec3 vary_normal;
 VARYING vec4 vertex_color;
 VARYING vec2 vary_texcoord0;
 
-vec2 encode_normal(vec3 n)
-{
-	float f = sqrt(8 * n.z + 8);
-	return n.xy / f + 0.5;
-}
+vec2 encode_normal(vec3 n);
 
 void main() 
 {