demo: fix creating WebGL textures from tiles

The cross-origin policy must be set.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>

Author: bgilbert

demo/index.html

@@ -240,6 +240,7 @@
             viewer = new OpenSeadragon({
                 id: "view",
                 prefixUrl: openseadragon_images,
+                crossOriginPolicy: "anonymous",
                 animationTime: 0.5,
                 blendTime: 0.1,
                 constrainDuringPan: true,