[mutter] wayland-dma-buf: doc the odd format mapping

[Ray Strode <halfline src gnome org>]

commit 63a6ead7d9449c10fdaeebcc3e13282dc70c1e23
Author: Pekka Paalanen <pekka paalanen collabora co uk>
Date:   Tue Oct 16 14:45:14 2018 +0300

    wayland-dma-buf: doc the odd format mapping
    
    This format mapping looks wrong at first hand, so explain why it is like
    it is.
    
    See also https://gitlab.gnome.org/GNOME/mutter/issues/323

 src/wayland/meta-wayland-dma-buf.c | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/src/wayland/meta-wayland-dma-buf.c b/src/wayland/meta-wayland-dma-buf.c
index 5f168f674..347da2484 100644
--- a/src/wayland/meta-wayland-dma-buf.c
+++ b/src/wayland/meta-wayland-dma-buf.c
@@ -3,6 +3,7 @@
 /*
  * Copyright (C) 2016 Red Hat Inc.
  * Copyright (C) 2017 Intel Corporation
+ * Copyright (C) 2018 DisplayLink (UK) Ltd.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -85,6 +86,13 @@ meta_wayland_dma_buf_buffer_attach (MetaWaylandBuffer *buffer,
 
   switch (dma_buf->drm_format)
     {
+    /*
+     * NOTE: The cogl_format here is only used for texture color channel
+     * swizzling as compared to COGL_PIXEL_FORMAT_ARGB. It is *not* used
+     * for accessing the buffer memory. EGL will access the buffer
+     * memory according to the DRM fourcc code. Cogl will not mmap
+     * and access the buffer memory at all.
+     */
     case DRM_FORMAT_XRGB8888:
       cogl_format = COGL_PIXEL_FORMAT_RGB_888;
       break;