Port to PHP 8

[duboism]

This PR ports the code to PHP 8: it now runs from PHP 5.6 to PHP 8.4.

The main changes to the code are:

• don't use curly braces for accessing array elements (0fd9ba2)
• some static methods were not declared as static (4e05aff and fb8a227)

This PR is based on the branch used for PR #3 (so this one must be merged first and I must rebase).

[CloCkWeRX]

Probably better to target https://github.com/pear/Image_Color2 but oh well we have merged newer content to this repo.

[duboism]

Probably better to target https://github.com/pear/Image_Color2 but oh well we have merged newer content to this repo.

I worked on this repo because pear/Image_Canvas uses it (and my project uses Image_Canvas). It was done at the beginning of 2024 (there was some exchange on the ML around that). I can have a look at pear/Image_Color2 too (but not right now).

[duboism]

Ah, thanks for solving the conflicts. This extra line is weird (probably a mistake on my side), I think we should remove it.

[duboism]

Thanks for your work (I was a bit busy today) and merging this.