OpenSlide 3.0.0

• Switch from GPLv2 to LGPLv2
• Reduce some unlikely memory leaks
• Support of more MIRAX files
• Improve performance of MIRAX rendering, vastly in some cases
• Reduce appearance of seams in MIRAX
• Add "quickhash-1" hash property
• Add openslide-quickhash1sum and openslide-show-properties tools
• Rework the API documentation
• Remove never-implemented prefetch functions from openslide.h (but
  retain with warnings in the library)
• Start attempting to figure out Trestle tile position files
• Add some defined property names to the header file