tests: make screenshooting return a buffer

Screenshooting does not involve creating a wl_surface, so using struct
surface is superfluous.

Return a struct buffer instead. It could have been just a
pixman_image_t, but setting up proper destruction would be a bit more
work. Should not hurt to keep the wl_buffer around until the user is
ready to free the image.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/tests/weston-test-client-helper.h b/tests/weston-test-client-helper.h
index adeeee3..794abb3 100644
--- a/tests/weston-test-client-helper.h
+++ b/tests/weston-test-client-helper.h
@@ -207,10 +207,7 @@
 pixman_image_t *
 load_image_from_png(const char *fname);
 
-struct surface *
-create_screenshot_surface(struct client *client);
-
-struct surface *
+struct buffer *
 capture_screenshot_of_output(struct client *client);
 
 #endif