fix cluster tile size calculation

[HackerFoo]

Objective

Fixes #4127 & #3596

Solution

1. Calculate how many tiles we can use per Z slice: n_tiles_xy
2. Then, estimate the size of each tile in pixels: px_per_tile
3. Number of tiles across, such that it's less that the screen width: n_tiles_x
4. Number of tiles down, such the the total is less that n_tiles_xy: n_tiles_y
5. Then, divide up the screen into tiles to get the actual dimensions of each tile, rounding up, so that we can cover the whole screen.

This, along with the new calculation in update(), should never fail the assertion.

Remaining tasks

• Add a unit test, either exhaustive up to 4k (12 + 12 bits = ~16M, should be pretty quick), or use proptest.

[alice-i-cecile]

I love property based testing, and would be happy to see it used in our code base.

[robtfm]

i've put in a PR (#4148) covering this ground on top of the latest updates. i should have commented on the issues sooner to save your time, sorry.

[HackerFoo]

i've put in a PR (#4148) covering this ground on top of the latest updates. i should have commented on the issues sooner to save your time, sorry.

No problem. I wrote this a while ago, so the only time lost was writing the description.