Compositor: New Posterize Node

The posterize node limits the number of colors per channel.
This is useful to generate masks or to generate stylized images

Both the tiled and full-frame implementation are included in this patch

{F10314012}

Reviewed By: manzanilla, jbakker

Differential Revision: https://developer.blender.org/D12304

source/blender/blenkernel/intern/node.cc

@@ -4919,6 +4919,7 @@ static void registerCompositNodes()
   register_node_type_cmp_inpaint();
   register_node_type_cmp_despeckle();
   register_node_type_cmp_defocus();
+  register_node_type_cmp_posterize();
   register_node_type_cmp_sunbeams();
   register_node_type_cmp_denoise();
   register_node_type_cmp_antialiasing();