Is there any way to get a 24-bit BGR ZPixmap from xcb_shm_get_image? All I can seem to get are 32-bit BGRA images, and that alpha channel seems really unnecessary. Editing the bitmask to exclude the alpha doesn't work, it only sets it to zero.
Well, for convenience 32 bits are always used when dealing with 24 bit color spaces. A CPU is much more efficient at handling 4 bytes at a time... The 'wasted' alpha byte is a good tradeoff between efficiency and memory consumption. So just copy the data yourself, packing the RGB bytes into 3 bytes per pixel.