made full-screen edge scrolling smoother and faster

made full-screen edge scrolling smoother and faster

The old method used a very slow frame rate with small steps,
which made it obnoxious during use.  The new method has a few
improvements:

- Calculates "edge" region width automatically, as a ratio of
  the viewport size.  Default is 1/16th.

- Uses a different edge width for x and y, which works better
  on very wide or very tall screens.

- Increased default frame rate from 10 fps to 60 fps.

- Replaced hardcoded frame rate with a #define.

- Reduced maximum movement per frame slightly, because the
  frame rate is so much faster.  The overall result is still much
  faster, but also easier to do small adjustments with.

- Fixed off-by-one error in scroll rate calculation formula.  Left/up
  scrolls were faster than down/right, but this is fixed.

This makes it more comfortable to use TigerVNC on a small screen
to work on a larger screen for hours at a time.