Brush randomization

[white-axe]

Connections

• Closes Random tile placement  #122

Description
This adds a checkbox to the top bar to make the brushes use random tiles from the rectangle of selected tiles in the tilepicker, which makes it easier to create randomly distributed tiles. Holding down the Alt key on the keyboard inverts the setting.

There's also a density setting in the top bar that allows making the brush only draw on a percentage of randomly chosen tiles on the map instead of every tile. This is intended to make it easier to create heterogeneous sets of random tiles, like if someone wants to create a map consisting mostly of one set of random tiles but with a small number of a different set of random tiles here and there.

Testing
Needs to be verified that the random patterns created by the Randomize ID setting and the density setting in the top bar are deterministic when drawing over the same region of a map multiple times, including with different brushes. This is important because otherwise the random tiles may not be uniformly distributed when the density setting is less than 1. Other than that, I highly doubt anything else needs to be tested.

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown -Z build-std=std,panic_abort
• Run cargo build --release
• If applicable, run trunk build --release