etcpak: Improve image padding and clean up the code

[BlueCube3310]

Changes the etcpak implementation's code to avoid resizing images whose resolutions aren't multiples of 4, and to only pad them instead. This improves import performance since the image only has to be adjusted once, and not resized.

Also cleans up the code.

[Calinou]

Tested locally, it works as expected.

This also makes textures sharper as they no longer need to be resized by a very slight amount (which affected sharpness negatively).

Testing project: test_pr_89567.zip

Before	After

Benchmark

Using an optimized editor build (optimize=speed lto=full). Import times are measured with --verbose CLI output.

When importing a 4K texture set such as this one, this is the time taken to import 6 textures in 4095×4095 size with VRAM compression:

Before	After (this PR)
5.31s	2.53s (2.1× faster)

With this PR, the import speed of non-multiple of 4 textures is now pretty much identical to the import time of the 4096×4096 textures (which have a size that's a multiple of 4).

PC specifications

• CPU: Intel Core i9-13900K
• GPU: NVIDIA GeForce RTX 4090
• RAM: 64 GB (2×32 GB DDR5-5800 C30)
• SSD: Solidigm P44 Pro 2 TB
• OS: Linux (Fedora 39)

[akien-mga]

Thanks!