[whatwg] hit regions: clearing of regions

[Rik Cabanier]

Currently, the specification states that if you create a region and then
create another region that completely covers region, the first region is
removed from the hit region list [1]

This is a complex operation that involves either drawing the regions to a
bitmaps and counting pixels, or path intersection.
It is also unintuitive because an author might expect that he could remove
the second region and still have the first region active.

What was the reason that this was added to the spec?

1:
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#clear-regions-that-cover-the-pixels