Fix TLS segfault in windows static build.

[mcmtroffaes]

I noticed a segfault when using avs+ windows static build, which I managed to reproduce with this simple sample program:

#include <iostream>
#include <avisynth_c.h>

int main(int argc, char **argv) {
    auto *env = avs_create_script_environment(3);
    if (env == nullptr) {
        std::cerr << "Failed to create AVS environment" << std::endl;
        return -1;
    }
    avs_delete_script_environment(env);
    return 0;
}

I traced the segfault to this location in the code:

https://github.com/AviSynth/AviSynthPlus/blob/master/avs_core/core/avisynth.cpp#L2224

The dwTlsIndex variable is set during DllMain:

https://github.com/AviSynth/AviSynthPlus/blob/master/avs_core/core/main.cpp#L285

which is obviously not called when the static library is built. The patch moves TlsAlloc/TlsFree to the constructor/destructor of a statically initialized object instead. A simpler solution might be to simply drop XP support and rely on __declspec(thread) instead, but I guess that might be more contentious.

[pinterf]

That XP_TLS can even be defined conditionally, triggered when v141_xp is chosen as platform, that is USING_V110_SDK71 is defined.

[mcmtroffaes]

That XP_TLS can even be defined conditionally, triggered when v141_xp is chosen as platform, that is USING_V110_SDK71 is defined.

Sure, I've added a commit to that effect, although I can't test it myself.

[mcmtroffaes]

Anything else needed here? Looking at the code again, I guess DllMain could be removed completely now if desired.

[pinterf]

I commited my XP_TLS usage proposal as a separate commit in 1eebe2e
Returning to the main part of the commit, its benefit would be to enable static build with XP compatible builds?

[mcmtroffaes]

I commited my XP_TLS usage proposal as a separate commit in 1eebe2e

Thanks!

Returning to the main part of the commit, its benefit would be to enable static build with XP compatible builds?

Yes, that's correct (or rather, fix a segfault that currently occurs when an XP compatible build is used; static builds are technically already possible).