Quick update to Image copies

Very small addition on the joys of VK_FORMAT_G8B8G8R8_422_UNORM

Author: spencer-lunarg

chapters/image_copies.adoc

@@ -232,8 +232,16 @@ VkBufferImageCopy region[2];
 region[0].imageSubresource.aspectMask = VK_IMAGE_ASPECT_PLANE_0_BIT;
 region[0].imageExtent = {width, height, 1};
 
-region[0].imageSubresource.aspectMask = VK_IMAGE_ASPECT_PLANE_1_BIT;
-region[0].imageExtent = {width / 2, height / 2, 1};
+region[1].imageSubresource.aspectMask = VK_IMAGE_ASPECT_PLANE_1_BIT;
+region[1].imageExtent = {width / 2, height / 2, 1};
 ----
 
-image::{images}image_copies_multi_planar.svg[image_copies_multi_planar.svg]
+image::{images}image_copies_multi_planar.svg[image_copies_multi_planar.svg]
+
+=== Singlar-Planar YCbCr
+
+There is anthoer edge case to watch out for! Formats such as `VK_FORMAT_G8B8G8R8_422_UNORM` can be tricky as they not are technically compressed, but also not multi-planar (so you copy with `VK_IMAGE_ASPECT_COLOR_BIT`). 
+
+The spec does give you a hint how to and handle them for copies:
+
+"[VK_FORMAT_G8B8G8R8_422_UNORM] For the purposes of the constraints on copy extents, this format is treated as a compressed format with a 2×1 compressed texel block."