[bug] Double buffering support not working

[hgruniaux]

Actually, double buffering should already be supported in my branch. BUT, it is not. The code is not working.

The idea, is to allocate a framebuffer twice the height of the requested size. And then use the SET_VIRTUAL_OFFSET tag (in mailbox) to change the "view" of the VideoCore into the framebuffer to implement the double buffering. However, tests show that do not work on QEMU.

The concept is exactly the one explained everywhere on forums. So, it should work. There is probably a problem in my code, or worse, QEMU may not support that. To be tested... on hardware.