Allow for querying supported sample counts in Msaa and expose 16 samples

[yzsolt]

Objective

Adds support for querying supported MSAA sample counts on the current GPU, making possible for users to select only supported MSAA modes, thus avoiding fatal WGPU validation errors. Also exposes MSAAx16, which wasn't available before.

• Fixes macos: anti_aliasing example crashes when MSAA is set to 8 samples #15789
• Closes MSAA with 8 samples crashes when using vulkan on windows 10 #3961
• Exposes MSAAx16 (related: More anti-aliasing options #14657)

Solution

1. At first I tried to put validation right before texture creation (TextureCache::get), since for some reason WebGPU ties max. sample count info to texture formats, and that's trivially available there. But max. sample count query also requires a RenderAdapter, and passing it to all callsites was not really feasible.
2. Then I realized that the underlying graphics APIs provide max. sample count on the device level, so we can probably get away with using a dummy/default texture format, so query/validation can be put into Msaa implementation instead.

• Msaa now exposes a method to retrieve supported sample counts on the given RenderAdapter. Usage is opt-in, Msaa::from_samples still won't check for actual GPU support.
• A utility constructor is now also available to create Msaa with the highest supported sample count.
• anti_aliasing example got adapted to these changes, to only show supported MSAA modes.

Testing

• Tested using the improved anti_aliasing example:
  • On an M1 MacBook it correctly displays only MSAAx2 and MSAAx4
  • On the web it correctly displays only MSAAx4
  • On a Windows laptop it correctly displays MSAAx2, MSAAx4 and MSAAx8, according to the iGPU's capabilities
• Not tested on MSAAx16 capable hardware (but there's not many such AFAIK anyway)

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[yzsolt]

I reverted the API breaking enum -> struct change, as I realized it was unnecessary. The changeset is much cleaner now, sorry about the previous noise.
I'm not 100% happy with the naming of SupportedMsaaList in the example, but I have no better idea, and it's probably not that important anyway.