Improved EXIF V3 compatibility

EXIF specs V3 added the UTF-8 data type. The MicroExif class now allows serializations to choose whether to support the V2 or V3 format. To maximize compatibility with old readers, even when V3 is set, the ASCII data type is used if possible.

The JXR plugin, based on TIFF V6 container, does not allow to use the V3 format (it does not recognize the UTF-8 data type) and therefore V2 has been forced. For all other plugins using MicroExif, it is now possible to save, e.g., descriptions in Japanese.

Please note that this patch is also a bugfix: when saving, version 3 was set but the strings were always saved as ASCII.

autotests/write/basic/avif.json

@@ -31,7 +31,7 @@
         },
         {
             "key" : "Description",
-            "value" : "TV broadcast test image."
+            "value" : "テレビ放送テスト映像。(TV broadcast test image.)"
         },
         {
             "key" : "Latitude",

autotests/write/basic/heif.json

@@ -31,7 +31,7 @@
         },
         {
             "key" : "Description",
-            "value" : "TV broadcast test image."
+            "value" : "テレビ放送テスト映像。(TV broadcast test image.)"
         },
         {
             "key" : "Latitude",

autotests/write/basic/jxl.json

@@ -31,7 +31,7 @@
         },
         {
             "key" : "Description",
-            "value" : "TV broadcast test image."
+            "value" : "テレビ放送テスト映像。(TV broadcast test image.)"
         },
         {
             "key" : "Latitude",