Disable half-drawn update mode when the center is moved.

Disable half-drawn update mode when the center is moved.

- When the center of the viewport is moved, all layers need to be redrawn.
- Data for half-drawn are now in VikWindow where they belong. Having had
  "trigger" as a static variable in viklayer would cause problems when
  viking has more than one windows.

Signed-off-by: Quy Tonthat <qtonthat@gmail.com>