Garbage mate input for keying node

This adds garbage matte input to new keying node which is used to
force occluding things which can not be eliminated by color operations.

White areas defines areas which should be removed from final result.

source/blender/nodes/composite/nodes/node_composite_keying.c

@@ -46,8 +46,9 @@
 /* **************** Translate  ******************** */
 
 static bNodeSocketTemplate cmp_node_keying_in[] = {
-	{	SOCK_RGBA, 1, "Image",			0.8f, 0.8f, 0.8f, 1.0f, 0.0f, 1.0f},
-	{	SOCK_RGBA, 1, "Key Color", 1.0f, 1.0f, 1.0f, 1.0f},
+	{	SOCK_RGBA,  1, "Image",	        0.8f, 0.8f, 0.8f, 1.0f, 0.0f, 1.0f},
+	{	SOCK_RGBA,  1, "Key Color",     1.0f, 1.0f, 1.0f, 1.0f},
+	{	SOCK_FLOAT, 1, "Garbage Matte", 0.0f, 1.0f, 1.0f, 1.0f},
 	{	-1, 0, ""	}
 };