Under Win16, I have a 65536-color or 256-color bitmap
(of size 32x32 because it's an icon's XOR mask) and I
want to convert it to a 16-color bitmap (I need to write
a 16-color XOR mask into a .ICO file). I am using Visual
C++ 1.52c under Windows 95/3.1.
What is the algorithm in C/C++ to do this?