textures.c: optimise PNG reading

[KrahJohlito]

Pull Request checklist

Note: these are not necessarily requirements

• [ ] I reformatted the code with clang-format
• [x] I checked to make sure my submission worked
• [x] I am the author of submission or have permission from the original author
• [ ] Requires update of the PS2SDK or other dependencies
• [ ] Others (please specify below)

Pull Request description

Reducing the number of operations should improve png loading times a bit