v0.10: Bugs, new shop, health regen

Good evening.

Bugs

1. The reroll button is now correctly disabled after the player buys an upgrade and dips their available gold below the reroll cost. Before, the button would remain enabled, but it didn't do anything.
2. The player ship will now correctly reverse its deterioration after sufficient healing. (If you haven't noticed, the ship sprite changes as you take more damage. This process can now reverse.)

Changes

1. The "Projectile Count" upgrade has had its cost reduced to be equal to the other upgrades in its rarities. I still think it's the best upgrade in the game, but with this many more upgrades available, the chance of it appearing is lower. The feeling of being able to afford any different Epic/Legendary, but not that one, is not good.
   1. The actual fix here is that I should make this upgrade come with a downside, like -25% damage or something, to balance it.
2. The shop now behaves pretty differently:
   1. There are four items on display, up from three.
   2. Purchasing an upgrade no longer refills that upgrade slot. The only way to change the upgrade offered in a given slot is to reroll, which resets all of them.
      1. The old methodology sometimes resulted in the cost of a reroll being higher than the cost of buying an upgrade, which meant it was sometimes correct to buy any random upgrade just to reroll that individual slot. No!
      2. The intention here is that you're shown 4 items, you buy the ones you want, and then you reroll to refresh all four slots.

Features

1. The game now has eight space nebula backgrounds that it chooses from for the menu, upgrade, and game screens.
2. I've added health regeneration and its corresponding upgrades.
   1. Common: 30g, 1 stack of regen
   2. Rare: 50g, 2 stacks of regen
   3. Epic: 75g, 3 stacks of regen
   4. Legendary: 100g, 5 stacks of regen

Health regen works like this: At the start of the round, I set a timer that heals the player for 1 health when it expires. The timer's wait time is calculated with the following formula:

3 / ( 1.0 + ( ( NUM_STACKS - 1) / 2.25 ) )

This is taken from Brotato, but modified slightly to be a little faster. All the relevant values are higher in my game -- player max health, damage received by enemies -- so I want our health regen to be higher. Here's a table of how much health you can expect to get back:

If these seem low, maybe they are! Only one way to find out.