[stella] INV+ Release Candidate 2

["Erik Mooney" <erik@xxxxxxxxxx>]

Changelog:

- "Fixed Manuel's bug" so the fire button starts a new game

- Implemented invisible invaders, since it sounded like Thomas
might do it himself if I didn't. :)  And it really was possible
in about 15 minutes with no extra RAM.  Hit Game Select twice to 
cycle into inviso mode.  Same rules as the Atari SI - the
invaders appear briefly when you hit one or get hit.

- The invaders' bombs are now always displayed in gray.  This is
a side-effect of the above - when I first made the invaders (playfield) invisible, the bomb (ball) also became invisible. :) 
When in inviso mode, the bombs cannot be seen until they fall 
below the lowest level of invaders - this is unavoidable.

- Added an easter-egg to display my initials.  Once you know
how, you can usually do it if you try, but I want to see if
anyone ever finds it. :)  (It's obfuscated in the source which
I'll post here eventually.)

I didn't think I was anywhere close to this, but I'm now also
out of ROM as well.  Of course, oodles of optimizations are
possible, but any new features now must really be worth that
effort. :)

- Erik

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