Improved Screenshot / Video capture

[ptheywood]

Providing keybindings to capture screen shots is potentially useful.

#43 adds very crude support for this, creating upside down bitmap named screenshot.bmp in the working directory, when P is pressed or the down arrow on a gamepad.

Improvements:

• A format other than bmp
• Not being upside down
• Sensible (unique) filenaming
• Error checking
• Periodic screenshots (once per N simulation steps?)
  • Potentially automatic video generation (or a post processing ffmpeg command)

[Robadob]

#49 solves the first two of these improvements.