Continuous camera zoom

[pjcozzi]

#2810 changed camera zoom to zoom to the point under the mouse cursor.

If the cursor is toward the bottom of the screen, this can still require several right click and drags before zooming all the way in. The zoom should continue as long as the right button is down even if the mouse is not moving, almost like the inertia for rotation.

We may want to make this an option (default true?), but let's experiment with it first.