Add windows association flags to WGPUSurfaceDescriptorFromWindowsHWND

[austinEng]

This PR google/dawn#16 adds DXGI_MWA_NO_ALT_ENTER always to Dawn.

Probably the better thing would be for an application to be able to configure the window association when creating the WGPUSurface.

Concretely, something like:

typedef struct WGPUSurfaceDescriptorFromWindowsHWND {
    WGPUChainedStruct chain;
    void * hinstance;
    void * hwnd;
+   uint32_t windowAssocationFlags;
} WGPUSurfaceDescriptorFromWindowsHWND WGPU_STRUCTURE_ATTRIBUTE;

For reference, valid flags are:

#define DXGI_MWA_NO_WINDOW_CHANGES  0x1
#define DXGI_MWA_NO_ALT_ENTER       0x2
#define DXGI_MWA_NO_PRINT_SCREEN    0x4

[kainino0x]

Jun 6 meeting:

• KN: Can the caller call MakeWindowAssociation themselves?
• AE: They don’t have the IDXGIFactory.
• KN: Could we pass it out to them, or have them pass it in?
• AE: Not sure, could cause layering violation problems.
• (looking through everything on IDXGIFactory through IDXGIFactory7)
• KN/AE: Seems fine to just pass this through for now and take a factory later in an extension or different descriptor, if it turns out to be useful
• AE: Name? mwa? mwaFlags?
• KN: I like the current name, mwa would be harder to look up
• Let’s do what’s written in the proposal

[Kangz]

I have slight concerns with adding the flags directly in this descriptor because it is only useful on the D3D11 and D3D12 backend, but the HWND descriptor can also be used to Vulkan and OpenGL/ES, so the flags would do nothing there. At the minimum we'll need docs to cover that.

[austinEng]

We hadn't considered HWND on Vulkan / GL - do you think this would be better as an independent feature?

I think if the feature isn't enabled, you'd get a validation error during surface.Configure(..)
(need to know the device first)

[kainino0x]

I guess if MakeWindowAssociation gets called in Configure we could make this an extension to Configure?

[austinEng]

Moving to an extension to Configure sounds fine to me. @SoapyMan would you like to champion adding this since it seems important for your use case?

[SoapyMan]

Sounds good to me

[kainino0x]

This has a resolution but it should be implemented before we try to standardize it. It's going to be an extension struct anyway so it's non-breaking (can be easily added after stabilization).