otTables: fix CompositeMode enum missing PLUS mode

Our enumeration of CompositeMode was missing one COMPOSITE_PLUS mode (12)...

see 'PaintComposite' paragraph in https://github.com/googlefonts/colr-gradients-spec/blob/main/OFF_AMD2_WD.md

Lib/fontTools/ttLib/tables/otTables.py

@@ -1480,21 +1480,22 @@ class CompositeMode(IntEnum):
 	SRC_ATOP = 9
 	DEST_ATOP = 10
 	XOR = 11
-	SCREEN = 12
-	OVERLAY = 13
-	DARKEN = 14
-	LIGHTEN = 15
-	COLOR_DODGE = 16
-	COLOR_BURN = 17
-	HARD_LIGHT = 18
-	SOFT_LIGHT = 19
-	DIFFERENCE = 20
-	EXCLUSION = 21
-	MULTIPLY = 22
-	HSL_HUE = 23
-	HSL_SATURATION = 24
-	HSL_COLOR = 25
-	HSL_LUMINOSITY = 26
+	PLUS = 12
+	SCREEN = 13
+	OVERLAY = 14
+	DARKEN = 15
+	LIGHTEN = 16
+	COLOR_DODGE = 17
+	COLOR_BURN = 18
+	HARD_LIGHT = 19
+	SOFT_LIGHT = 20
+	DIFFERENCE = 21
+	EXCLUSION = 22
+	MULTIPLY = 23
+	HSL_HUE = 24
+	HSL_SATURATION = 25
+	HSL_COLOR = 26
+	HSL_LUMINOSITY = 27
 
 
 class PaintFormat(IntEnum):