dnd: work around cairo-gl brokenness

If cairo-gl is used, display_create_surface() will create an
wl_egl_window for each surface and this will result in errors if this
surface is used as a source. Also, one can't get a wl_buffer for such
a surface wich led to crashes when trying to do so for the drag icon.

This patch works around both problems by forcing the item and drag icon
surfaces to use shm.

clients/dnd.c

@@ -106,7 +106,8 @@ item_create(struct display *display, int x, int y, int seed)
 
 	rect.width = item_width;
 	rect.height = item_height;
-	item->surface = display_create_surface(display, NULL, &rect, 0);
+	item->surface =
+		display_create_surface(display, NULL, &rect, SURFACE_SHM);
 
 	item->x = x;
 	item->y = y;
@@ -322,7 +323,8 @@ create_drag_cursor(struct dnd_drag *dnd_drag,
 
 	rectangle.width = item_width + 2 * pointer->width;
 	rectangle.height = item_height + 2 * pointer->height;
-	surface = display_create_surface(dnd->display, NULL, &rectangle, 0);
+	surface = display_create_surface(dnd->display, NULL, &rectangle,
+					 SURFACE_SHM);
 
 	cr = cairo_create(surface);
 	cairo_translate(cr, pointer->width, pointer->height);

clients/window.c

@@ -618,7 +618,7 @@ display_create_surface(struct display *display,
 	if (check_size(rectangle) < 0)
 		return NULL;
 #ifdef HAVE_CAIRO_EGL
-	if (display->dpy)
+	if (display->dpy && !(flags & SURFACE_SHM))
 		return display_create_egl_window_surface(display,
 							 surface,
 							 flags,

clients/window.h

@@ -112,6 +112,7 @@ display_get_image_for_egl_image_surface(struct display *display,
 #endif
 
 #define SURFACE_OPAQUE 0x01
+#define SURFACE_SHM    0x02
 
 cairo_surface_t *
 display_create_surface(struct display *display,