Fixed saving TIFF images to wxMemoryOutputStream.

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