External texture

[Kangz]

FYI @litherum @cwfitzgerald

The external texture descriptor is pretty Dawn-specific but the rest seems like it could be shared easily. WDYT?

[litherum]

• Should probably say that this fixes ExternalTexture support #188
• I think most of WGPUExternalTextureDescriptor is Dawn-specific, so it probably should be moved into a chained struct, rather than being in the top-level WGPUExternalTextureDescriptor struct. For comparison, our implementation of WGPUExternalTextureDescriptor is much simpler. Also, for us, we can use the platform-specific CVPixelBufferRef which includes a lot of the metadata within itself.

Other than the above, this looks awesome; thanks for adding support!