This was using screens[1] which I had deallocated when fixing the status
bar (I incorrectly assumed it was used only for that).
While the CGDOOM technique to share screens[1] to avoid allocating the
320x20 buffer for the status bar makes clear sense with that new
information, I think I'll keep this 6.4 kB buffer there and rather
search for ways to use more memory zones.
* Restore screen numbers; BG is 4, at least in the ST module.
* Let ST module allocate BG, which is just 32 pixels high and not a full
VRAM (huge memory gain!)
* Fix V_CopyRect() not working because memcpy is still broken (this will
be changed later with a proper memcpy)
Lephenixnoir
ComputerNerd