fbdev: first version of double buffering
This version creates 2 pixman_image_t on the video memory if it's possible.
During the rendering from one to the other
fbdev: second version of double buffering
this version create 2 pixman_image_t with the video memory.
One is used for the screen rendering, the next is used for the shadow rendering.
After each rendering completion shadow and hardware image are switched.
fbdev: enable the double buffering if the video memory size is enought