fix: conversion of AVIF image rotation property to EXIF orientation

This implements a fix for the issue identified in
AOMediaCodec/libavif#2727 and fixed in AOMediaCodec/libavif#2729. The
code to convert irot and imir properties to EXIF orientation when
decoding AVIF images in Pillow was repurposed from libavif, so it
suffers the same bug.

Author: fdintino

src/_avif.c

@@ -59,7 +59,7 @@ irot_imir_to_exif_orientation(const avifImage *image) {
                 return axis ? 7   // 90 degrees anti-clockwise then swap left and right.
                             : 5;  // 90 degrees anti-clockwise then swap top and bottom.
             }
-            return 6;  // 90 degrees anti-clockwise.
+            return 8;  // 90 degrees anti-clockwise.
         }
         if (angle == 2) {
             if (imir) {
@@ -75,7 +75,7 @@ irot_imir_to_exif_orientation(const avifImage *image) {
                            ? 5   // 270 degrees anti-clockwise then swap left and right.
                            : 7;  // 270 degrees anti-clockwise then swap top and bottom.
             }
-            return 8;  // 270 degrees anti-clockwise.
+            return 6;  // 270 degrees anti-clockwise.
         }
     }
     if (imir) {