window: Let clients set buffer transformations

When a window's buffer transformation is set, its buffers are
reallocated with the appropriate size (i.e., with width and height
swapped in case of 90 or 270 degree rotation).
diff --git a/clients/window.h b/clients/window.h
index 804eef5..f13ea35 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -240,6 +240,13 @@
 window_show_frame_menu(struct window *window,
 		       struct input *input, uint32_t time);
 
+int
+window_get_buffer_transform(struct window *window);
+
+void
+window_set_buffer_transform(struct window *window,
+			    enum wl_output_transform transform);
+
 void
 window_destroy(struct window *window);