webglapp: blockUI when re-shading
webglapp: blockUI when re-shading

Can take a couple of seconds when computing betweenness centrality
for hundreds of skeletons.

fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID,htem/CATMAID,fzadow/CATMAID