Missile explosion tweaks

Added a contra explosion sprite sheet, and a way to define the explosion
to use for missiles. Tweaked explosion render timing.

Also added chiptune theme music.
Render tweaks to enemies, bugged

Figuring out how to render a moving entity and scale it correctly.

Will now do a series (25) of ray traces per enemy in view to render it
in stripes. Some stripes may be hidden behind walls, others can be seen.

Few issues at the moment. One is bug in math that renders stripes too
far apart. Have to render them close together to look like one image,
and stay that way through scaling for distance.

Another is scaling is off in general. Render of enemy will shift to the
side even when backing up while centered on it. Should scale while
centered if player is approaching/retreating dead on.

Also need to sort entities by z order before each render cycle, but that
is an easy addition once the above is fixed. Only difficulty may be
sorting two different set of entities, e.g. enemies and missiles at the
moment. Wonder if I can keep a render array that combines both, or a
super entity type and entity array.