Implement slow GD test helpers in C

[cmb69]

There are two test helpers which need to traverse all pixels of the given images, what is obviously rather slow in PHP. To avoid that performance penalty, we implement the functions in C and make them available as internal PHP functions, but only if GD_TEST_HELPERS is defined when building. We still keep the PHP implementations as fall- back in case someone wants to run the tests without the internal functions being defined.

We also improve the userland functions by traversing the pixels in Z order what is more cache efficient for the libgd implementation.

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TODO:

• use internal functions for all CI jobs?
• choose better names (maybe prefix with gd_test_

[cmb69]

No, this doesn't make sense. At best, we can save a couple of seconds for "normal" test runs (assuming we do some performance optimization on the userland test helpers; I'll follow up on that soon), and that doesn't outweigh the cost of having to maintain two sets of test helpers.

[thg2k]

Maybe #14914 would be helpful. I'd be happy to rework it.

[cmb69]

@thg2k, possibly. However, it is not clear what exactly we should expose to userland (discussion apparently went nowhere), and it is indeed hard to come up with a general solution. I'll try to have a closer look at #14914 and #14877 and the related discussions (https://externals.io/message/124306 and https://externals.io/message/124383, respectively).