add some upgrade comments (#457)

these document improvements and/or removal of unnecessary code for when
we will require a higher minimum version of Imlib2.

all the comments have been prefixed with "UPGRADE: " for easy grepping.

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/457
Reviewed-by: explosion-mental <explosion-mental@noreply.codeberg.org>

image.c

@@ -563,6 +563,8 @@ Imlib_Image img_open(const fileinfo_t *file)
 	{
 		imlib_context_set_image(im);
 	}
+	/* UPGRADE: Imlib2 v1.10.0: better error reporting with
+	 * imlib_get_error() + imlib_strerror() */
 	if (im == NULL && (file->flags & FF_WARN))
 		error(0, 0, "%s: Error opening image", file->name);
 	return im;
@@ -581,6 +583,7 @@ bool img_load(img_t *img, const fileinfo_t *file)
 	 */
 	imlib_image_set_changes_on_disk();
 
+/* UPGRADE: Imlib2 v1.7.5: remove these exif related ifdefs */
 /* since v1.7.5, Imlib2 can parse exif orientation from jpeg files.
  * this version also happens to be the first one which defines the
  * IMLIB2_VERSION macro.

nsxiv.h

@@ -159,7 +159,7 @@ typedef keymap_t button_t;
 
 /* image.c */
 
-#ifdef IMLIB2_VERSION
+#ifdef IMLIB2_VERSION /* UPGRADE: Imlib2 v1.8.0: remove all HAVE_IMLIB2_MULTI_FRAME ifdefs */
 	#if IMLIB2_VERSION >= IMLIB2_VERSION_(1, 8, 0)
 		#define HAVE_IMLIB2_MULTI_FRAME 1
 	#endif

thumbs.c

@@ -268,6 +268,7 @@ bool tns_load(tns_t *tns, int n, bool force, bool cache_only)
 			char tmppath[] = "/tmp/nsxiv-XXXXXX";
 			Imlib_Image tmpim;
 
+			/* UPGRADE: Imlib2 v1.10.0: avoid tempfile and use imlib_load_image_mem() */
 			if ((ed = exif_data_new_from_file(file->path)) != NULL) {
 				if (ed->data != NULL && ed->size > 0 &&
 				    (tmpfd = mkstemp(tmppath)) >= 0)