Fix incorrect tile UVs when using atlas feature on AMD hardware

[rparrett]

Fixes an issue reported by @odecay on discord.

This is specific to AMD hardware and the issue and fix were reproduced by @odecay and @bzm3r. I am unable to reproduce on any of my non-AMD HW.

Here's a changeset on their fork that reproduces the issue:
main...odecay:bevy_ecs_tilemap:main

That example should show something like this:

But shows this instead:

It seems that there's some sort of floating point imprecision thing going on where var columns ends up as 2 rather than the correct 3 after being cast as u32 / implicitly floored. This messes up the rest of the UV calculations.

It is possible to induce this same visual result on hardware that doesn't exhibit this bug by subtracting 1 from var columns.

[bzm3r]

Big if true! And it is true!

[odecay]

Can confirm this change fixes the issue for me 👍