Support Multi-GPUs

[yyc-git]

Now I have one computer(win10 os) with multiple gpus(e.g. two RTX 2070s), How to use all of them to render???

Thanks very much!!!

[yyc-git]

I asked in gpuweb/gpuweb#995 also:

Using multiple GPUs separately (getting multiple GPUAdapters and creating a GPUDevice on each one) would be enabled by some API changes I intend to propose eventually (pre-1.0), but there is no API surface or implementation for it yet.

it said that it will be implement in pre-1.0