Add support for very basic mouse-drag scrolling

Mouse-drag scrolling simulates touch scrolling for devices without
touchscreens. It will be useful for performance work on a variety of
devices and a stopgap for interaction until scrollbars exist.  The
implementation is very basic now, with support for rudimentary
rate-limiting, but will need deeper integration into the compositor for
proper iframe/overflow:scroll support.
In any case I would move this to the top of the file.