Make badguys' pattern not random-based (WIP)

[Semphriss]

(Edit:) Resolves #959

A badguy that's fun to beat should have a well-defined, easy-to-understand pattern so that new players that aren't well used to the game dont get surprised by a sudden, unexpected change of movement from a badguy.

It also helps speedrunners by not throwing a whole run simply because mister pilot decided it'd hover at the worst height possible and stay there on the best run of the day.

Currently:
Pilots accelerate randomly. Each time you load a level, the pilot's pattern will be different.

At the moment, this PR makes it so that:
Pilot have a deterministic, but hard-to-predict pattern. That is, playing the same level and going at the same pace should result in similar encounters with the badguys. This solves the speedrunners' problems, but does not yet solve the predictability problem.

To-do:

• Make the pilot/flying snowball follow a simple, clearly-defined movement. It could be a simple sine wave movement, or something a bit more complex like this : https://www.desmos.com/calculator/aw4dpgqntf (if link dies, the formula is sin(x)^3 + sin(3 * (x - pi/3))/3)
• Investigate and remove randomness from the skullyhops, I've been told they have random movements too.

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I opened this draft PR so that people could share their thoughts about what should be done and how. Above are my opinions, but I'd like to hear about other team members, level designers, speedrunners and players too.

[weluvgoatz]

Very nice indeed!
The name "pilot" is cringe, blech :P

[Semphriss]

To-do complete!

The pull request is ready for review!

[serano01]

Very well, I've tested it and will now merge it. Thanks!