add SurfaceCapabilities to PixelsContext

[alago1]

Small change to expose the SurfaceCapabilities struct used when building the Pixels instance through the PixelsContext.

This should allow conditionally setting wgpu PresentMode based on the available options for the Pixels surface (Issue #423).

[parasyte]

Thank you for the contribution!

[parasyte]

It looks like this will technically allow the developer to "fix up" PresentMode after the surface has already been configured.

Another option is to do the obvious thing when creating the surface: If the chosen PresentMode is not already in the list of capabilities, replace it with the first one available. Any other complex fallback logic can be delegated to user code with the new PixelsContext::surface_capabilities field. WDYT? I'm ok with having that as a separate PR, too.

[alago1]

That makes sense. I added another commit to this PR which handles this and clarifies the language in the PixelsBuilder::present_mode to be an 'optimistic setter' rather than providing any guarantees about the state once the surface is actually built.

Also, I used AutoVsync as the default case since it's always supported (wgpu docs) and is what's currently set by Pixels anyway.

[parasyte]

Sorry for the delay. I’m returning home from vacation right now. The change looks good and I’ll get this merged as I find time.