This makes two-finger scrolling in straight lines easier, while still
allowing free/diagonal movement. It works in three stages:

1) Initial movement
   - For the first few millimeters, scroll movements within 30 degrees
     of horizontal or vertical are straightened to 90-degree angles.
   - Scroll movements close to 45 degree diagonals are unchanged.
   - If movement continues very close to straight horizontal or
     vertical, stage 2 begins and the axis lock engages.
   - If movement continues along a diagonal, stage 2 is skipped and
     free scrolling is immediately enabled.
2) Axis lock
   - If the user scrolls fairly closely to straight vertical, no
     horizontal movement will happen at all, and vice versa.
   - It is possible to switch between straight vertical and straight
     horizontal, and the axis lock will automatically change.
   - If deliberate diagonal movement is detected at any point, stage
     3 begins and the axis lock disengages.
3) Free scrolling
   - Scrolling is unconstrained until the fingers are lifted.