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.