I got the tiling-without-scaling-the-image part sorta working. It now crops the original image to dimensions that are as close to the original as I know how to get them, rather than scaling the image and crunching the pixels. Unfortunately this only works properly for images that are no larger than the screen. It seems fl_read_image, when applied to an on-screen image, only reads what's visible onscreen (including any external windows that might be in front of it), and leaves the rest of the area black.

So I guess I do need to learn how to use fl_read_image with an off-screen buffer. This might be tricky, since fl_create_offscreen doesn't exist in murgaLua. I think I'm going to need help learning how murgaLua.createOffscreenBuffer works.