patch [#32195] MASKS: Canonical Porter Duff algorithm for merge missing.

from Troy Sobotka (sobotka)

This gives nicer blending then 'ADD', setting as default for new masks.

source/blender/blenkernel/intern/mask.c

@@ -156,6 +156,7 @@ MaskLayer *BKE_mask_layer_new(Mask *mask, const char *name)
 
 	mask->masklay_tot++;
 
+	masklay->blend = MASK_BLEND_MERGE;
 	masklay->alpha = 1.0f;
 
 	return masklay;

source/blender/blenkernel/intern/mask_rasterize.c

@@ -1251,6 +1251,9 @@ float BKE_maskrasterize_handle_sample(MaskRasterHandle *mr_handle, const float x
 		}
 
 		switch (layer->blend) {
+			case MASK_BLEND_MERGE:
+				value += value_layer * (1.0f - value);
+				break;
 			case MASK_BLEND_ADD:
 				value += value_layer;
 				break;

source/blender/makesdna/DNA_mask_types.h

@@ -173,7 +173,8 @@ enum {
 	MASK_BLEND_DARKEN      = 3,
 	MASK_BLEND_MUL         = 4,
 	MASK_BLEND_REPLACE     = 5,
-	MASK_BLEND_DIFFERENCE  = 6
+	MASK_BLEND_DIFFERENCE  = 6,
+	MASK_BLEND_MERGE       = 7
 };
 
 /* masklay->blend_flag */

source/blender/makesrna/intern/rna_mask.c

@@ -573,6 +573,7 @@ static void rna_def_maskSpline(BlenderRNA *brna)
 static void rna_def_mask_layer(BlenderRNA *brna)
 {
 	static EnumPropertyItem masklay_blend_mode_items[] = {
+		{MASK_BLEND_MERGE, "MERGE", 0, "Merge", ""},
 		{MASK_BLEND_ADD, "ADD", 0, "Add", ""},
 		{MASK_BLEND_SUBTRACT, "SUBTRACT", 0, "Subtract", ""},
 		{MASK_BLEND_LIGHTEN, "LIGHTEN", 0, "Lighten", ""},