Fix area splitting from action zone flipping viewports

Fix area splitting from action zone flipping viewports

The top-left and bottom-right corners were creating the new area in the
wrong place.

Blender 2.7x only had action zone corners in the top-right, and
bottom-left corners. So it had some hardcoded assumptions based on that.

This commit feels a bit like a hack, but I think it may be fine.

Bug reported via IRC, how to reproduce:

* Change shading to Rendered.
* Split viewport from the top-left corner.