[Merged by Bors] - fix cluster tiling calculations

[robtfm]

Objective

fix cluster tilesize and tilecount calculations.
Fixes #4127 & #3596

Solution

• calculate tilesize as smallest integers such that dimensions.xy() tiles will cover the screen
• calculate final dimensions as smallest integers such that final dimensions * tilesize will cover the screen

there is more cleanup that could be done in these functions. a future PR will likely remove the tilesize completely, so this is just a minimal change set to fix the current bug at small screen sizes

[superdump]

I think we should probably be using debug_assert for situations that could happen due to programming errors within bevy that would cause runtime problems, but that if we wrote our code properly, should never happen. And then use assert for things that could happen at runtime in release builds due to a user error, though proper error handling would be nicer than panicking... so I'm trying to think of a situation where assert should really be used... I realise I've used assert elsewhere in this code but the thought came up now. @cart @mockersf do you have any opinion / clear thoughts and guidelines on this?

[cart]

I think we should probably be using debug_assert for situations that could happen due to programming errors within bevy that would cause runtime problems

The rule of thumb I've been using:

1. When possible, use tests instead of asserts. The more expensive an assertion is, the more you should try to make it a test. We shouldn't meaningfully compromise dev performance because we aren't confident in our code. For expensive checks, we should achieve confidence through other forms of testing.
2. For cheap assertions that can't be tests, use debug_assert.

[cart]

This particular case doesn't feel like a debug_assert. This is a user-facing type that has constraints on the values it supports. These constraints should either be enforced here (panic if unrecoverable, or warn + auto-constrain to avoid crashing) or they should be enforced via a constructor. I'm inclined to auto-constrain and warn here.

[cart]

bors r+

[bors]

Pull request successfully merged into main.

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