Fix integer underflow when rounding up in VoxelGI

[darksylinc]

The code wanted to divide and round up:

• 0 / 64 = 0
• 63 / 64 = 1
• 64 / 64 = 1
• 65 / 64 = 2

However when the dividend was exactly 0 it would underflow and produce 67108864 instead.

This caused TDRs on empty scenes or extremely slow performance

Fix #80286

[lawnjelly]

Given that this is used in multiple places, and this error has creeped in (probably via copy paste), would this be better to fix once and use a macro or inlined math function in the different uses? 🤔

e.g. #define GODOT_ROUND_UP(N, S) ((((N) + (S) - 1) / (S)) * (S))

(and / or version without the multiplier as you are using here)

[Calinou]

Note that doesn't appear to resolve #55359. (I tested this as requested in #80286 (comment).)

OS: Linux
CPU: Intel Core i9-13900K
GPU: GeForce RTX 4090
Resolution: 3840×2160

All tests done with VoxelGI quality set to High in the project settings.

Mode	Before (master)	After (this PR)
Dynamic + Static	358 FPS (2.79 mspf)	355 FPS (2.81 mspf)
Dynamic only	359 FPS (2.78 mspf)	356 FPS (2.80 mspf)
Static only	524 FPS (1.90 mspf)	523 FPS (1.91 mspf)
Neither dynamic nor static	258 FPS (1.89 mspf)	528 FPS (1.89 mspf)

[AThousandShips]

I believe you have a transposition error in row four column one, 258 instead of 528

[clayjohn]

Looks great!

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.2.