Successfully locate MacPorts-installed OpenSlide
Update to current OpenSeadragon

Closes #7.
Don't show extra border or allow zooming out further than required
Drop workaround for timer leak in OpenSeadragon < 0.9.125
Change tile fetch timeout from 5 seconds to 2 minutes

Tile generation sometimes takes longer than 5 seconds.
Make full-screen viewer use the whole screen
index.html: Drop viewer border when full-screen
index.html: Reinstate some configuration from older OpenSeadragon

which used to be the default.  Allow zooming to 2x the highest-resolution
level, and load an extra level of tiles for improved image quality.