Bug fix #7: Correct a possible issue in VSync

- I realized I might have fundamentally misunderstood VSync. This commit
  tries to fix the possible problem, or at least move the tearing line
  close to the top of the screen.

- Software VSync is replaced by --sw-opti (software optimization), as
  I guess it isn't possible at all to do VSync without driver support.

- Add "vsync" and "sw-opti" as configuration file options.

compton.sample.conf

@@ -34,7 +34,9 @@ mark-ovredir-focused = true;
 detect-rounded-corners = true;
 detect-client-opacity = false;
 refresh-rate = 0;
+vsync = "none";
 paint-on-overlay = true;
+sw-opti = false;
 
 # Window type settings
 wintypes: