[gimp] app: add a comment to test-xcf.c about why we use LEGACY layer modes

[Michael Natterer <mitch src gnome org>]

commit 1cbb2f3309c6090d0583c17392975aa86e52a321
Author: Michael Natterer <mitch gimp org>
Date:   Sun Aug 20 14:29:56 2017 +0200

    app: add a comment to test-xcf.c about why we use LEGACY layer modes

 app/tests/test-xcf.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/app/tests/test-xcf.c b/app/tests/test-xcf.c
index 6343fdd..df0a993 100644
--- a/app/tests/test-xcf.c
+++ b/app/tests/test-xcf.c
@@ -58,6 +58,11 @@
 #include "gimp-app-test-utils.h"
 
 
+/* we continue to use LEGACY layers for testing, so we can use the
+ * same test image for all tests, including loading
+ * files/gimp-2-6-file.xcf which can't have any non-LEGACY modes
+ */
+
 #define GIMP_MAINIMAGE_WIDTH            100
 #define GIMP_MAINIMAGE_HEIGHT           90
 #define GIMP_MAINIMAGE_TYPE             GIMP_RGB