Block Visibility with Context and Zen

Turns out that if you're using Context with a Zen 2.x-based subtheme, any block visibility rules you set up with Context get blown away by Zen. The reason is that both Context and Zen implement the `theme_blocks()` function, but Zen, being unaware of Context, clobbers all the work done by Context.

The solution is pretty simple: apply this patch to Zen. Note that you must be using at least Context 3.x for the patch to work; otherwise, you'll get a WSOD.

And for those who are interested, here's some background on the issue.