Fixed saving TIFF images to wxMemoryOutputStream.

Libtiff attempts to seek past the end of a stream and the behaviour for this can vary per stream implementation. Fixed failure to seek by filling the gap between the end of stream and new seek position with zeroes. Enabled a unit test which so far was disabled due to wxMemoryOutputStream failing to save a TIFF because of the seeking problem.

Also closes #4089.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68772 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/image/image.cpp

@@ -912,15 +912,8 @@ void CompareImage(const wxImageHandler& handler, const wxImage& image,
         return;
     }
 
-    if (type == wxBITMAP_TYPE_JPEG /* skip lossy JPEG */
-        || type == wxBITMAP_TYPE_TIF)
+    if (type == wxBITMAP_TYPE_JPEG /* skip lossy JPEG */)
     {
-        /*
-        TIFF is skipped because the memory stream can't be loaded. Libtiff
-        looks for a TIFF directory at offset 120008 while the memory
-        stream size is only 120008 bytes (when saving as a file
-        the file size is 120280 bytes).
-        */
         return;
     }