[x] Scale damage based on distance. This should apply to both player shots and enemy shots
[x] Shots at very close range are 'critical.' Add a double green blink for those, and regenerate a little bit of health
[x] Move all constants to #defines so that Dac can tweak balance
[x] Double enemy speed on all difficulties
[x] +20% player speed on hard mode
[x] Add invincibility frames to enemies after being shot so they can't be stunlocked. Flicker-invert the sprite as long as invincibility frames are active.
[x] Add a warning light (yellow) before an enemy attacks
[x] Double tap left/right to strafe. Strafing will dodge an attack. Must be done in conjunction with the warning light
#defines
so that Dac can tweak balance