SVS import and TIFF export fixes

Magnification is now preserved when reading SVS files.  /cc @aleksandra-tarkowska

Various issues with TIFF export, and with JPEG-2000 compression in particular are now resolved.  JPEG-2000 boxes can now be stripped before being saved in a TIFF, and truncated strips/tiles are appropriately padded.  The TIFF writer is also now suitable for being subclassed.

/cc @chris-allan