During "make updatebootstrap", we use less than 0x20 bytes on the
PSP side and less than 0x40 bytes on the RSP one. 0x100 bytes ought
to be enough for anybody.

I implement the screen using XCB which is much more friendly
than z80e's SDL+CMake for development machines that want to install
minimal dependencies (for example, a port-less OpenBSD rig).

This gives the maximum size of the stack at any given moment during the
execution of the program. It's useful to figure out if the stack gets
dangerously close to the heap.