Add const qualifier to SurfaceCapabilities lists

[eliemichel]

The end user of GetCapabilities should not be allowed to mutate capabilities as far as I understand it.

[Kangz]

I'm not sure: the structure returned and its memory is owned by the user of the API so they can mutate it if they'd like.

[eliemichel]

True, though imho non-const pointers somehow suggest that the user could mutate the actual capabilities by mutating the fields of the WGPUSurfaceCapabilities struct.

And I can hardly imagine a use case where one would need to mutate these... but on the other hand why not if we do not need to forbid it.

[kainino0x]

Jan 4 meeting:

• KN: Wondering if these could ever actually be owned by some other object and not allocated just for the WGPUSurfaceCapabilities object?
• CF: Could have refcounting underneath, then it could be shared owned.
• CF: Is const_cast undefined behavior in C++?
  • KN: In this case I’m pretty certain it would be.
  • (So it would be OK to return a const pointer to internal data.)
• …
• KN: Think they should be const in case of the refcounting situation or something.
  • CF: Actually doing this requires some tricks so FreeMembers can get at the refcount from just the SurfaceCapabilities struct. But possible.
• Let’s make them const