v0.4.2: Boss Scaling and Shield Rework

We are so back.

I am home from a nice vacation down at the beach and I am working on my little game. Fire walk with me (haven't seen it).

1. The Round 20 version of the boss is now stronger than the Round 10 version:
   1. its HP is doubled
   2. the chance that it fires a burst of projectiles is 1/3rd, up from 1/5th
   3. the number of waves in its burst fire is 6, up from 4
2. I have reworked the Shield a little bit. The pickup is identical (same drop rate, same despawn timer, same Ability behavior) but the way the Shield actually functions is a little different.
   1. Before, the Shield would absorb one single instance of damage and then disappear. In later rounds, this was effectively useless due to how many small, relatively low-damage bullets are on the screen.
   2. Now, the Shield has a "HP" value, and if you are shielded and receive damage, the Shield absorbs that damage before it moves on to your ship. The value for this HP is 20% of your ship's max HP. (This value scales as you upgrade your HP.)
      1. I think this is a buff to all characters (even for the Mosquito, the HP here is 8, which is right on par with Round 1 projectiles).
      2. This is a bigger buff to the Tank, clearly, since the Shield there starts at 30HP(!) and increases even faster due to the percentage-based upgrades. I do not know that the Tank needs this buff.
      3. I think the Shield needs a buff like this, though, so let's give this a shot for now.

What's Next?

I am approaching the point here where I am considering the core game systems to be mostly generally ready to go, broadly, if you don't squint too much. I need to (and want to) add a lot more content like new Ships, Abilities, Weapons, Upgrades, Bosses, all that stuff. But that feels like it's in a different bucket than anything to do with the core game loop.

The game desperately needs new custom-made art, sounds, and music. This is now the first thing on my list of priorities.