UPDATE: SEEMS TO BE BROKEN!
Scratch or Turbowarp probably updated something.
OLD BLURB:
turbowarp.org/561537878?fps=60&hqpen
Reliably detect when Turbowarp is fullscreen and HD pen is enabled.
To use, take the get_graphics_quality custom block and put it in your backpack.
Note that it erases all pen, so you may want to use it before your main render script.
Also, the background must not be black (i.e. the same colour as the hd_detector sprite).