Converts an RGB value into the current pixel format.
Converts colors from a hardware independent format (red, green, and blue
values ranging 0-255) to the pixel format required by the current video
mode, calling the preceding 8, 15, 16, 24, or 32-bit makecol functions as
/* Regardless of color depth, this will look green. */
int green_color = makecol(0, 255, 0);