only remove main image (ifd0) orientation tag

we were stripping all orientation tags on autorot

see https://github.com/libvips/libvips/issues/1865

libvips/conversion/autorot.c

@@ -9,6 +9,9 @@
  * 10/5/20
  * 	- handle mirrored images
  * 	- deprecate vips_autorot_get_angle()
+ * 24/10/20
+ * 	- only remove main image orientation, since we don't rotate the
+ * 	  embedded thumbnail
  */
 
 /*
@@ -69,8 +72,7 @@ static void *
 vips_autorot_remove_angle_sub( VipsImage *image, 
 	const char *field, GValue *value, void *my_data )
 {
-	if( vips_isprefix( "exif-", field ) &&
-		vips_ispostfix( field, "-Orientation" ) ) {
+	if( strcmp( field, "exif-ifd0-Orientation" ) == 0 ) {
 #ifdef DEBUG
 		printf( "vips_autorot_remove_angle: %s\n", field ); 
 #endif /*DEBUG*/