Partial transparency, chromakey, and opacity!

This update brings a lot of novelties!

• There is now a function that enables per-pixel partial transparency!
  The GIF that you should be seeing above uses it to create a stylized character that can walk around on your desktop. “desktop_friend.zip” is the executable for it and “desktop_friend.yyz” is the source code.
  Shoutout to Penzilla for the character sprite. Kind of hard to find a neat non-pixelart character for this sort of showcase, believe it or not.
• There is now a function for chromakey transparency!
  It’s a cheap, simple option to complex shapes with sharp edges.
• There is now a function to adjust window alpha!
  This is a simple multiplier that works with other functions.