This was missing a wrapper.

I had issues with 128bytes because I wasn't passing an 128 byte image
to begin with (somehow 32x32 turnd into 32x41)... So that works.

Commenting to make it clearer both what is being done (manual creation
of an 128 byte mask, with all bits set, a.k.a full visibility).

And lastly convert source to namestring since sdl-load-bmp only
accepts a string, and I didn't think about it until I tested with namestrings...