Add JXR support to the browser

Can be enabled/disabled at runtime by toggling the 'media.jxr.enabled'
pref (disabled by default).

Two additional prefs are provided for testing purposes:
'media.jxr.autoaccept', and 'media.jxr.advertised_mime_type'
See comments in all.js for information on what these do.

This commit includes the MS OpenTech implementation of the decoder on
the browser side with some fixes applied; see the development in Pale
Moon GitGub issue #105 or my comments in the source code for more
information.

image/src/Image.cpp

@@ -50,6 +50,17 @@ Image::GetDecoderType(const char *aMimeType)
   else if (!strcmp(aMimeType, IMAGE_JPG))
     rv = eDecoderType_jpeg;
 
+#ifdef MOZ_JXR
+  // JXR (JPEG XR)
+  else if (
+      !strcmp(aMimeType, IMAGE_JXR) || !strcmp(aMimeType, IMAGE_MS_PHOTO)
+  ) {
+    if (gfxPrefs::MediaJXREnabled()) {
+      rv = eDecoderType_jxr;
+    }
+  }
+#endif
+
   // WEBP
   else if (!strcmp(aMimeType, IMAGE_WEBP))
     rv = eDecoderType_webp;