[webappsec] New SVG examples for UISecurity obstruction check

I've added some drawings in my working copy of UISecurity to give examples
of calculating the boundaries of an obstruction check.  I'd appreciate any
comments anyone would like to offer before I incorporate them into the
shared editor's draft.

Do people think they are helpful?

http://webappsec-test.info/~bhill2/user-interface-safety/user-interface-safety.html#example-boundary-calculations-for-the-obstruction-check

I've also added the following to the alternative heuristic:

"As a short-cut, a user agent may choose to treat any pixels in a protected
layer with an opacity of less than 100% as failing to match by definition.
In cases where a fully-composited user view is not available or extremely
expensive to calculate, this optimization allows the obstruction check to
be performed with only a knowledge of the layers that fall on top of the
protected layer."

-Brad