Keep resize cursor better while dragging

[srguiwiz]

Hitherto when carefully dragging column splitbar to the left or row splitbar to the top then cursor exiting splitbar proper changed from resize to other.

That was visually annoying, flickering.

This fix makes the cursor remain resize much better.

By saying "mouse is in middle of splitbar" it stays within splitbar better.

Have verified it still correctly respects min-size.

[SomeKittens]

Plunk implementing this code: http://embed.plnkr.co/tGgrojQnYyV74QI8ajvL/preview (thanks @j--w )

[SomeKittens]

I'm still seeing the cursor type flicker back and forth. Am I missing something?

[j--w]

Its less flickering when dragging slowly, don't think its different enough to be considered a fix though.