XENON 1 remake by JOTD

Based on the 1983 oric game by John S Sinclair, IJK Software,
and when you play it a little bit, looks like a blatant rip-off of Phoenix, of another game I don't know the name (for the parachutists stage), but who cares?
it still rules.

features:

- pixel perfect rendering of the original game
- original algorithms used for enemy moves
- multi-key controls (which is one of the advantages on playing the original
  on any emulator)
- graphical enhancements (backgrounds)
- new sounds

options:

-d: double display
-e: enable enhanced gfx features
-f: full screen
-j: enable joystick

keys:

ESC: quit
ARROWS: move left/right
CONTROL: fire
ARROW UP: disengage (when too few fuel)
P: pause

joystick:

button 1: fire
button 2: disengage

history:

v1.1

- darker planet background for better contrast
- slightly slower meteorites level
- fixed some joystick issues
- other misc fixes

v1.0

- double screen mode (using -d option)
- enhanced mode (using -e option, forces -d option)
- full screen mode (using -f option)
- pause key
- joystick support (-j option)
- custom sound support
- hiscore load/save
- fixed hiscore entry (original rubout bug)

v0.1

- no sound
- basic mode
- remaining bugs
