Cannot compile for iOS

[jinleili]

When config my app's cargo dependencies to latest wgpu-rs commit,compiling to iOS failed:

but compiling to macOS was successful.

[seivan]

@grenlight Maybe you need a --features metal?
It shouldn't afaik drag in gfx-backend-vulkan for iOS & macOS.

[jinleili]

the latest commit removed backend features, so cannot use --features metal