• Fix crash on launch on Linux and Windows with "GLFW.createWindow failed to create a … window".
  GLFW rejected the requested OpenGL 2.1 context, since it was combined with a profile hint which is only defined for OpenGL 3.2 and above.
  The AppImage of version 1.1 was affected on every Linux distribution.
• Smoke test the AppImage in CI by starting the GUI on a virtual X server, so crashes on launch are caught before a release
• Scale the interface with the display scaling setting, instead of drawing it at a fixed pixel size.
  It used to come out at a third of its intended size on a high DPI display set to 175 % scaling.
  The scaling can be overridden with the PERSPEC_UI_SCALE env variable.
• Shrink the initial window if it wouldn't fit on the screen
• Only start the GUI when perspec.exe is run without arguments from a console of its own, i.e. was double-clicked.
  Running it without arguments in a terminal prints the usage text again.