Add wgpu multiviewport support

[Zelif]

Supersedes #7132

Add Multiviewport support to the WebGPU backend and example.

Tasks outstanding:

• Have user defined callback for creating a surface when create window is called
• Remove wgpu_instance & headers needed for surface creation
• Change the format and presentation mode better (select preferred rather than first available but fall back to first available)
• Fix present mode assert to use ~0 not 0

Note

Linux users(using Dawn):
Due to an intel bug on linux/vulkan, the Mailbox present mode is the only thing available using vulkan backend. This disables Vsync. Using both AMD/Nvidia seems fine to edit dawn's code directly but that isn't a solution.
The old manual swapchain creation and usage and selecting FiFo is still working.
Fixed with this commit:
https://dawn.googlesource.com/dawn/+/780bbd6fce6b8e81569406fdb4b490613d158733%5E%21/#F2

[Zelif]

@ocornut & @eliasdaler
Unsure if that is how you would want the callback, but seems to work.

The changes that have been done are:

• To move away from Swapchain and use SurfaceConfigure . (Dawn has finally updated to be inline with wgpu-native)
• Add a callback for the user to define how to handle the surface creation after the window has been made
• Updated the example to include the callback using the glfw surface creation helper so the wgpu impl can be independent. (If SDL was used per say they could create the surface from the platform handle if that is what they are using)

Hopefully this sorts out multiviewport for desktop versions.

Warning

It seems to hang when trying to clean up the surface in some sort of race condition I assume. If I put a breakpoint inside the surface release code and resume the program hang is gone. I think the vulkan surface gets cleaned up before the internal swapchain gets deleted, this results in the program hanging.

This issue seems to not happen on my windows machine, as it seems to be an issue with the vulkan part.

Oddly enough it only happens when I drag the child window in and drop it, if I use the main window to encompass the child window there is no hang/crash.

[Zelif]

This might be questioned, so I will explain this.
This gets the first available texture format and presentation mode for the surface, on linux current with dawn since there is no FiFo presentation mode for vulkan the example will not work for linux.
This solves it, it will get the first available presentation mode which in most system the first one is FiFo and on linux will be Mailbox.

[ocornut]

Hello,

If you have time would mind:

• rebasing this on latest.
• squashing, or splitting into multiple commits if there is are possible distinct changes to make.
• confirming that this compile with all 3 targets (Emscripten, WGPU? Dawn?) and which of them may support multi-viewport (obv not Emscripten). As you now we now use IMGUI_IMPL_WEBGPU_BACKEND_DAWN/IMGUI_IMPL_WEBGPU_BACKEND_WGPU define to support those targets which haven't converged.

Thank you!

[ocornut]

About the WGPUSurface (*CreateViewportWindowFn)(ImGuiViewport* viewport) = nullptr; function:

The Vulkan backend has been facing a similar problem, which I have solved with a function pointer in ImGuiPlatformIO:

int (*Platform_CreateVkSurface)(ImGuiViewport* vp, ImU64 vk_inst, const void* vk_allocators, ImU64* out_vk_surface); // (Optional) For a Vulkan Renderer to call into Platform code (since the surface creation needs to tie them both).

In our case this is filled by the platform backend:

platform_io.Platform_CreateVkSurface = ImGui_ImplSDL3_CreateVkSurface;

// Vulkan support (the Vulkan renderer needs to call a platform-side support function to create the surface)
// SDL is graceful enough to _not_ need <vulkan/vulkan.h> so we can safely include this.
#include <SDL3/SDL_vulkan.h>
static int ImGui_ImplSDL3_CreateVkSurface(ImGuiViewport* viewport, ImU64 vk_instance, const void* vk_allocator, ImU64* out_vk_surface)
{
    ImGui_ImplSDL3_ViewportData* vd = (ImGui_ImplSDL3_ViewportData*)viewport->PlatformUserData;
    (void)vk_allocator;
    bool ret = SDL_Vulkan_CreateSurface(vd->Window, (VkInstance)vk_instance, (VkAllocationCallbacks*)vk_allocator, (VkSurfaceKHR*)out_vk_surface);
    return ret ? 0 : 1; // ret ? VK_SUCCESS : VK_NOT_READY
}

I would be open to potentially e.g.

• add a dedicated function in PlatformIO (but its signature would need to be portable, so not use WGPU type)
• see if we can merge the functions (if it makes sense)

But your solution may the best one because in the case of wgpu the desirable function is simpler to implement.

[Zelif]

* rebasing this on latest.

* squashing, _or_ splitting into multiple commits if there is are possible distinct changes to make.

Will do.

* confirming that this compile with all 3 targets (Emscripten, WGPU? Dawn?) and which of them may support multi-viewport (obv not Emscripten). As you now we now use IMGUI_IMPL_WEBGPU_BACKEND_DAWN/IMGUI_IMPL_WEBGPU_BACKEND_WGPU define to support those targets which haven't converged.

I will have a look over it and see what changes need sorting, I am quite behind with the WGPU releases(last I touched it was these changes)