Implement double buffering in simple buffer manager

[tehKaiN]

Double buffering is needed for drawing if you don't want to race the display beam.

Many years ago I had some clever idea of implementing double buffering as vport manager which could work regardless if one uses simple or scroll buffer, but right now I have no idea how that could work. The only remnant of this idea is in viewport manager priority list:

/**
 *  Viewport manager IDs.
 *  Number determines processing order. Camera is last, so rest may see
 *  a difference between its current and previous position
 */
#define VPM_SCROLL       0
#define VPM_TILEBUFFER   1
#define VPM_DOUBLEBUFFER 2
#define VPM_CAMERA       128