Indicator in Image Editor of which polygon you're texture painting on in 3d View

If you are painting in the 3d view it is often useful to know which polygon you are painting on in the 2d window, especially if the painting is going bad and you want to debug the problem, eg, give a polygon more UV space.

It'd be convenient if there were an indicator in the 2d spacethat would move around as you painted in 3d space, which is often difficult to discern the location of, esp. with more subtle color contrasts.

I suggest a circle the same size as the current Brush, rendered in XOR fashion so it would always contrast with whatever colors the user is using.