Shader cleanup - applies DRY to various shaders in the game (#23294)

shader cleanup - applies DRY to various shaders in the game

Resources/Textures/Shaders/greyscale.swsl

@@ -1,6 +1,5 @@
 void fragment() {
     highp vec4 color = zTexture(UV);
-    highp float grey = dot(color.rgb, vec3(0.299, 0.587, 0.114));
 
-    COLOR = vec4(vec3(grey), color.a);
+    COLOR = vec4(vec3(zGrayscale(color.rgb)), color.a);
 }