Blender Git Loki

View Navigation: 2D viewport zooming

This implements the same concept of 2D viewport panning but for zooming.
It prevents unexpected zoom speeds when working with a pen tablet.
Zoom speed is constant and it is based on the initial distance to the
working area. Speed parameters are hardcoded for now, but they can be
exposed as options or replaced by a variable zoom speed.
I also reduced the rotation speed when 2D viewport panning is enabled
for consistency.
This navigation mode is enabled by activating 2D viewport panning in the
navigation preferences. That option should be renamed in the future.
Enabling and disabling perspective is broken most of the time, but it
also happens in master. I'm not sure if these changes are making the
problem worse.