XGetImage returns junk with Xorg/dummy_drv/VTs

[Alan Cox]

> Because when you don't have the VT, you don't have the hardware, so you
> can't submit commands to it.  We don't preserve the framebuffer when we
> VT switch away, we just generate exposures for all windows when we
> restore X.
> 
> There was some debate about this way way back in like xfree86 4.0 or so.
> I don't remember what the motivation was for what we've got now, but
> it's what we do, so...

Speed of switching for one. Nothing like copying several megabytes of
data from a PCI/AGP card via PIO to make life suck.

Presumably GetImage should either block or cause a redraw into an
offscreen pixmap ?