ABI version number in GPUInstanceDescriptor

[kainino0x]

@Kangz suggests that we could allow applications to opt into breaking changes to the ABI after 1.0, by accepting a version number in GPUInstanceDescriptor that tells us the major version of webgpu.h being used. This would for example:

• if the application passes version 1, implementation accepts the original struct definition, which can take a chained struct for some core thing that got added after 1.0
• if the application passes version 2, implementation accept a new struct definition that folds the chained struct in

This would be pretty lightweight to reserve for future use so I think we should just add it.

[austinEng]

This would be nice for #284 if for example, it becomes core. Then we wouldn't need the chained struct for something that eventually will always available.

[kainino0x]

Apr 11 meeting:

• Could hide it inside the header, like have wgpuCreateInstance be inline and call an internal interface with an ABI version
• KN: Don’t necessarily have to actually do this
• AE: Would very much like to
• KN: What about function argument lists, structs-by-value?
  • structs by value include: FreeMembers, callback structs
• RM: if we take version in instance descriptor, can’t change instance descriptor
  • could take it as a separate parameter outside the instance descriptor
• Why do GL/Vk use function pointers? Remove an indirection through the loader
• Could just deal with this in GetProcAddress, e.g. by exposing v2 symbols
• Can add GetProcAddress2 or CreateInstance2 after 1.0 if we want
  • Implement this before trying to standardize it