WindowServer: Coordinate double-buffering with the BochsVGA card.

Use the BochsVGA card's virtual-height + virtual-y features to implement
a "hardware double buffering" type scheme.

This is a performance degradation since we now draw a bunch more than before.
But there's also no tearing or cursor flickering. I'm gonna commit this and
try to improve upon it. :^)

Kernel/BochsVGADevice.h

@@ -15,6 +15,7 @@ public:
 
     PhysicalAddress framebuffer_address() const { return m_framebuffer_address; }
     void set_resolution(int width, int height);
+    void set_y_offset(int);
 
 private:
     void set_register(word index, word value);