OpenSlide 3.4.0

• Major internal restructuring
• New formats: Hamamatsu NDPI, Sakura SVSLIDE, Ventana BIF (preliminary)
• Add openslide_detect_vendor()
• Deprecate openslide_can_open() (not very useful and often misused)
• Document performance considerations for openslide_open()
• Add properties giving the bounds of the non-empty region of the slide
• leica: Support multiple main images if their levels are coplanar
• leica: Use slide size as level size
• mirax: Support PNG- and BMP-formatted slides
• mirax: Fix "Expected first 0 value" error opening some slides
• mirax: Fix incorrect tile placement on some slides without overlaps
• mirax: Never synthesize downsampled levels
• Add OPENSLIDE_DEBUG environment variable (OPENSLIDE_DEBUG=? for help)
• Fix some crashes in error paths
• Add tests for many error paths