OpenSlide 3.3.0

• Support Leica SCN format (requires libtiff >= 4) (thanks, Agelos Pappas)
• Allow opening MIRAX 2.2 slides (though there are seams, bug #92)
• Add standardized microns-per-pixel and objective-power properties
• Add "macro" associated image in Trestle
• Rename "layer" to "level" throughout the API (deprecate "layer" functions;
  remove "layer" properties)
• Report parse errors in openslide_open() by returning an openslide_t in
  error state
• Deprecate openslide_get_comment()
• Add openslide_get_version()
• Improve command-line tools; add manpages
• Support building with MinGW-w64; drop CMake, MSVC, mingw32
• Add tests for many error paths