Hey Jim,
It was, but I made this file a year and a half ago so i found much better ways since then to approach stuff like this and this may not work in current browsers now.
If i was to make this file now i would use
to make the mask, and i would make the svg mask follow the cursor.. Sorry this file is a bit outdated..
The modified file I just posted above would work though, it's using the mask uedvincent did and it just moves it with the setElementProperty call on drag.