Here's the corrected version of the patch.
Henrik, not only the patch shouldn't bring about any sigsegv on your machine
but your Flat program should perfectly work in windowed mode: Allegro now
tries first to allocate the special backbuffer in video memory and, if it
can't, allocates it in system memory. There's no performance decrease since
this backbuffer isn't used when the window is in the foreground.
However, let me know...
Isaac, I also added a pair of _enter_gfx_critical() / _exit_gfx_critical()
statements to the windowed driver update functions. I think this should get
rid of the freezes I sometimes (very rarely) experienced with the GUI in
windowed mode.