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/BKE_node.h

@@ -1255,6 +1255,7 @@ void ntreeGPUMaterialNodes(struct bNodeTree *localtree,
 #define CMP_NODE_DENOISE 324
 #define CMP_NODE_EXPOSURE 325
 #define CMP_NODE_CRYPTOMATTE 326
+#define CMP_NODE_POSTERIZE 327
 
 /* channel toggles */
 #define CMP_CHAN_RGB 1