shell: Enhance the basic random positioning algorithm

Place the window in a random position on the output where the first seat with
a pointer is. When calculating the random position limit the range to the
area that would ensure that the whole surface is visible. If the surface is
larger than the output then the surface is placed at the origin of the
output.

This change is based on the good work of Scott Moreau <oreaus@gmail.com>

src/shell.c

@@ -2590,6 +2590,68 @@ center_on_output(struct weston_surface *surface, struct weston_output *output)
 				 surface->buffer->height);
 }
 
+static void
+weston_surface_set_initial_position (struct weston_surface *surface,
+				     struct desktop_shell *shell)
+{
+	struct weston_compositor *compositor = shell->compositor;
+	int ix = 0, iy = 0;
+	int range_x, range_y;
+	int dx, dy, x, y, panel_height;
+	struct weston_output *output, *target_output = NULL;
+	struct weston_seat *seat;
+
+	/* As a heuristic place the new window on the same output as the
+	 * pointer. Falling back to the output containing 0, 0.
+	 *
+	 * TODO: Do something clever for touch too?
+	 */
+	wl_list_for_each(seat, &compositor->seat_list, link) {
+		if (seat->has_pointer) {
+			ix = wl_fixed_to_int(seat->pointer.x);
+			iy = wl_fixed_to_int(seat->pointer.y);
+			break;
+		}
+	}
+
+	wl_list_for_each(output, &compositor->output_list, link) {
+		if (pixman_region32_contains_point(&output->region, ix, iy, NULL)) {
+			target_output = output;
+			break;
+		}
+	}
+
+	if (!target_output) {
+		weston_surface_set_position(surface, 10 + random() % 400,
+					   10 + random() % 400);
+		return;
+	}
+
+	/* Valid range within output where the surface will still be onscreen.
+	 * If this is negative it means that the surface is bigger than
+	 * output.
+	 */
+	panel_height = get_output_panel_height(shell, target_output);
+	range_x = target_output->current->width - surface->geometry.width;
+	range_y = (target_output->current->height - panel_height) -
+		  surface->geometry.height;
+
+	if (range_x < 0)
+		dx = 0;
+	else
+		dx = random() % range_x;
+
+	if (range_y < 0)
+		dy = panel_height;
+	else
+		dy = panel_height + random() % range_y;
+
+	x = target_output->x + dx;
+	y = target_output->y + dy;
+
+	weston_surface_set_position (surface, x, y);
+}
+
 static void
 map(struct desktop_shell *shell, struct weston_surface *surface,
     int32_t width, int32_t height, int32_t sx, int32_t sy)
@@ -2609,8 +2671,7 @@ map(struct desktop_shell *shell, struct weston_surface *surface,
 	/* initial positioning, see also configure() */
 	switch (surface_type) {
 	case SHELL_SURFACE_TOPLEVEL:
-		weston_surface_set_position(surface, 10 + random() % 400,
-					    10 + random() % 400);
+		weston_surface_set_initial_position(surface, shell);
 		break;
 	case SHELL_SURFACE_FULLSCREEN:
 		center_on_output(surface, shsurf->fullscreen_output);