Improve pattern generator

- Changed the delimiter from ";" (semicolon) to "|" (horizontal pipe)
- Allow image URLs in place of color definitions as well
- Position must now be specified as relative coordinates 0..1 just like for the Argyll dispcal/dispread -P parameter
- Size must now be specified as scaling factor just like for the Argyll dispcal/dispread -P parameter
- Default size when no scaling factor given is fullscreen proportional stretch to cover whole screen
- Size with scaling factor 1 is the same as a 256x256 pixel square on a screen with a horizontal resolution of 1920 pixels

All these are now valid:
  "http://localhost/example.png" (just a fullsize PNG)
  "http://localhost/example.png||.5|.5|1" (centered at default size)
  "http://localhost/example.png||.5|.5|1|1" (centered at non-proportional size)
  "http://localhost/example.png|#f00|.5|.5|1" (red background)