Trackpad: make Clip/Image editor respect the Zoom Axis setting.

Make Clip/Image editor respect the Zoom Axis setting. **
Make zoom speed more consistent with View2D & 3D.
Correct handling of the Invert Zoom options.

** The reason for this is that when you move diagonally (top-left <> bottom-right)
it is simultaneously an increase and a decrease, so it doesn't work very well.