Raft: Fix item prefilling

[SunnyBat]

What is this fixing or adding?

Raft is prefilling frequencies incorrectly when multiple Raft worlds with different frequencies are used in the same multiworld. This makes Raft worlds use their own items instead of any Raft world's items.

Currently, what Raft's prefilling is doing is it's looking for the first item with a given name in the item pool. When every Raft player in a multiworld has similar settings (in this case, everyone either has prefilled items or does not have prefilled items), this is not a problem -- players' items are inserted into the item pool and removed from the item pool in (hopefully) the same order, so their generations are correct. However, when there are different settings in different Raft worlds, problems arise. Now, the Raft worlds that prefill items are at risk of using other Raft worlds' items, since they're pulling the first item with a given name, rather than guaranteeing their own item.

This does three things:

1. Correctly prefills frequencies on all Raft worlds, even with different settings
2. Puts progressive-frequencies into the duplicate item pool. It reduces the odds of getting a progressive-frequency when filling the items (compared to any frequency in a non-progressive frequency mode) to avoid spamming a ton of progressive-frequency items.
3. Tracks the items that will be prefilled per-world, meaning that we don't need to look through the entire multiworld's item pool to pull out the items we care about. This is likely a performance win, especially on larger multiworlds.

How was this tested?

Manually tested multiple different Raft configurations with multiple different settings for frequency placement in multiple multiworld generations, and verified that each Raft world's locations were in an expected spot.
I also mixed up the order of the frequency placements to verify that it wasn't just a "lucky order" in which players' items were prefilled.

[Berserker66]

Suggested change

	if self.multiworld.island_frequency_locations[self.player] == 0: # Vanilla
	if self.multiworld.island_frequency_locations[self.player] == "vanilla":

for future reference, you can just do that.