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.

Author: bgilbert

examples/deepzoom/templates/index.html

@@ -120,6 +120,8 @@ <h1>Slide properties</h1>
         animationTime: 0.5,
         blendTime: 0.1,
         constrainDuringPan: true,
+        maxZoomPixelRatio: 2,
+        minPixelRatio: 0.5,
         minZoomLevel: 1,
         visibilityRatio: 1,
         zoomPerScroll: 2,