colors use more memory than neccessary

[Brian-ED]

colors are from 0 to 255 but bqn stores arrays as signed ints so it should really be -128 to 127 to save 2x space

[Brian-ED]

an issue i've found now is that i can't change the ffi functions themselves to output signed ints when there are colors as output, and i also can't change ffi functions to have color inputs to functions being signed, and reason for this is i can't reasonably implement it long term since every raylib update could add another function that uses color. What would be ideal is if the rayffi parser could automatically find where colors are and make them signed but that doesn't seem possible.

So, only thing to do is alter only the src/image.bqn to use signed ints and just expect the user to use the image.bqn code for loading images instead of using the ffi functions manually.