Changes to the EarthManipulator to allow zoom and pinch at the same time.

Removed the need to check for the previous event type in multitouch and
check with time.  Works better with GraphicsWindowWin32 now.
Removed the need to set the OPTION_SCALE_Y for dragging for multitouch
to be sensible.  This may break Android dragging if it isn't set.
Merge branch 'master' of https://github.com/gwaldron/osgearth