native: Ability to set a WSI programmatically about dxvk

[flibitijibibo]

At one point I thought about making it accessible via the DXGIFactory since we need to know which WSI to use the moment anything DXGI gets touched, but it's worth noting that a dxvk-native application must set the variable on startup, so if the application and its launcher both use it they're required to specify the WSI as if they were the base process (we treat "no value found" as a hard error to avoid apps depending on any kind of default value), so that would avoid the conflict in this particular example.

A real export wouldn't be so bad though, an example could be tried with this line in FNA3D: https://github.com/FNA-XNA/FNA3D/blob/master/src/FNA3D_Driver_D3D11.c#L5198

[doitsujin]

Introducing an entry point for this sounds good, definitely cleaner than the env var we currently have.

[flibitijibibo]

I'd be okay with something like extern "C" int dxvkSetWSI(const char*); in libdxgi; as long as it's easy to dlsym it shouldn't be too hard to add to existing D3D backends!

[smcv]

I'd be okay with something like extern "C" int dxvkSetWSI(const char*); in libdxgi; as long as it's easy to dlsym it shouldn't be too hard to add to existing D3D backends!

That sounds great to me, but I don't know the D3D/DXGI architecture well enough to know where best to put the global variable that would be set by this function, such that there would only be one copy of it per process and it would be accessible to everything that needs to know which WSI is the active one. (The obvious candidates seem to be static libraries that I think get linked more than once, making them unsuitable for that purpose.)

[flibitijibibo]

That's a good question; I believe WSI instances occur in both DXGI and D3D9/8 devices; the part that's tough is that the very first API call for each library will want WSI immediately so it's tough to add it to existing interfaces... @Joshua-Ashton, any ideas for D3D9 in particular?

[smcv]

Is there a "bottom-level" library that all of the others call into (is that DXGI?), like the way everything in the GTK ecosystem depends on GLib? If there is, then it could have an extern "C" int dxvkSetWSI(const char*) and a matching extern "C" const char *dxvkGetWSI(void).

But if there's no obvious bottom-level library then it might have to be more like dxvkDXGISetWSI, dxvkD3D9SetWSI, dxvkD3D8SetWSI.

[flibitijibibo]

The latter route is probably the way to go - the bottomest(?) library is libdxvk for DxvkInstance but that's statically linked, so entries for each lib is probably all we can do.