BIG DOG — two versions on this image.

    bigdog          play the COMPILED-RENDERER version (SPACE or W flaps)
    bigdog-demo     same, flying itself, for watching the frame rate
    tclsh /home/bigdog.tcl      the original, Tcl draws every pixel

WHAT CHANGED
The game logic is still Tcl. The pixels are not: /bin/fbdraw is a static
wasm32 C binary that owns /dev/fb0, and Tcl describes each frame to it as a
few dozen command lines over a pipe.

Measured in one boot, 300 frames at 480x320, under the Node runner:

    home/bigdog.tcl          (Tcl draws every pixel)   602 ms/frame
    home/bigdog-fbdraw.tcl   (fbdraw renders)          2.1 ms/frame

The old one was never limited by the framebuffer — the device does 367 MB/s.
It was limited by an interpreter touching pixels one list element at a time.

The new version asks fbdraw for the screen geometry instead of assuming it.
That matters here: the browser framebuffer is 640x480 with a stride of 7684
bytes (sized for the maximum mode, not 640*4), while bigdog.tcl has 7680
baked in as a literal.

WHY IT IS FAST
mmap(/dev/fb0) fails ENODEV here, and pwrite64 is ENOSYS on the wasm32
syscall table, so a frame costs lseek+write syscalls. fbdraw keeps the last
flushed frame and writes only rows that actually changed; a dog crossing a
static sky dirties a fraction of the screen and the rest cost a memcmp.

Source: demos/bigdog-fbdraw.tcl and packages/custom/fbdraw/fbdraw.c