Indicate tiles currently being processed in the 2D Viewer

[bentoogood]

Primarily of use with the Display node to show buckets being evaluated by the renderer.

part of #73

[johnhaddon]

The tiles being computed by a 3d renderer are actually entirely different to the tiles being processed by the 2D Viewer - and the chances are that the two have totally different tile sizes (fixed in 2d and based on bucket size in 3D).

To implement this I think we'd need a custom overlay/manipulator associated with the Display node, which displays the information coming from the 3d render. But before we can even do that we need to support the passing of "I'm about to render this bucket" messages from the renderer to our IECore::DisplayDriver, which is not yet possible. I'm not even sure 3delight has such a message...

[bentoogood]

Sub ticket of #73