IE < 10 does not support gradients.

Since the new imgdiff UI heavily used gradients, this leaves IE9 and
IE8 users with a complete disfunctional UI.

Replaced all gradients by simpler means:

* Vertical handle: instead of 2px wide gradient, use a 1px background
  and a white border-right
  
* Opacity slider: No gradient but a simple border and background-color

* All radial-gradients replaced by small pre-computed PNGs

* Checkerboard background done with a small repeated PNG

With these changes, the imgdiff UI is fully functional even in IE 8.
Visual changes in other browsers are barely discernible.