From the todo:

New render output option "No Output", which renders without
forcing an editor to show an image. Nice for people who
prefer to setup composites with background image view.

source/blender/editors/render/render_internal.c

@@ -197,6 +197,9 @@ void screen_set_image_output(bContext *C, int mx, int my)
 	SpaceImage *sima;
 	int area_was_image=0;
 
+	if(scene->r.displaymode==R_OUTPUT_NONE)
+		return;
+	
 	if(scene->r.displaymode==R_OUTPUT_WINDOW) {
 		rcti rect;
 		int sizex, sizey;

source/blender/makesdna/DNA_scene_types.h

@@ -883,7 +883,8 @@ typedef struct Scene {
 #define R_OUTPUT_SCREEN	0
 #define R_OUTPUT_AREA	1
 #define R_OUTPUT_WINDOW	2
-/*#define R_OUTPUT_FORKED	3*/
+#define R_OUTPUT_NONE	3
+/*#define R_OUTPUT_FORKED	4*/
 
 /* filtertype */
 #define R_FILTER_BOX	0

source/blender/makesrna/intern/rna_scene.c

@@ -1949,6 +1949,7 @@ static void rna_def_scene_render_data(BlenderRNA *brna)
 		{R_OUTPUT_SCREEN, "SCREEN", 0, "Full Screen", "Images are rendered in full Screen"},
 		{R_OUTPUT_AREA, "AREA", 0, "Image Editor", "Images are rendered in Image Editor"},
 		{R_OUTPUT_WINDOW, "WINDOW", 0, "New Window", "Images are rendered in new Window"},
+		{R_OUTPUT_NONE, "NONE", 0, "No Output", "Images are rendered without drawing"},
 		{0, NULL, 0, NULL, NULL}};
 	
 	/* Bake */