OpenSlide 3.2.3

• Support MIRAX files that use a variant format for tile
  positions (thanks, Hauke Heibel and Marco Feuerstein)
• Update location of website
• Add background color property, for slides that have it
• Update CMake scripts and other Windows fixes
  (thanks Hauke and Marco)
• Fix some test.c bugs
• Fix incorrect MIRAX drawing at certain resolutions
  (thanks Hauke and Marco)
• Support quickhash-1 on older systems (thanks, Jan Harkes)