Explain why avifImageCopy() disallows self copy

src/avif.c

diff --git a/src/avif.c b/src/avif.c
index 0da12c2..76185db 100644
--- a/src/avif.c
+++ b/src/avif.c
@@ -250,6 +250,8 @@
 
 avifResult avifImageCopy(avifImage * dstImage, const avifImage * srcImage, avifPlanesFlags planes)
 {
+    // Disallow self copy even though it could be supported easily. Self copy is
+    // unlikely to be needed, so it almost always indicates a programming error.
     AVIF_CHECKERR(dstImage != srcImage, AVIF_RESULT_INVALID_ARGUMENT);
     avifImageFreePlanes(dstImage, AVIF_PLANES_ALL);
     avifImageCopyNoAlloc(dstImage, srcImage);