Enable Vulkan for WGPU runner (#852)

* Enable Vulkan for WGPU runner

Metal doesn't support SPIR-V shader passthrough, so on macOS using WGPU runner you get following error with any example:

```
Features SPIRV_SHADER_PASSTHROUGH are required but not enabled on the device
```

Vulkan supports shader passthrough, but requires compatibility layer of MoltenVK to work, which needs to be installed on the host machine. Currently this feature is only available on master.

* Update examples Cargo lockfile

Author: andrusha

Cargo.lock

@@ -19,7 +19,8 @@ use-compiled-tools = ["spirv-builder/use-compiled-tools"]
 cfg-if = "1.0.0"
 shared = { path = "../../shaders/shared" }
 futures = { version = "0.3", default-features = false, features = ["std", "executor"] }
-wgpu = { version = "0.12", features = ["spirv"] }
+# Vulkan SDK or MoltenVK needs to be installed for `vulkan-portability` to work on macOS
+wgpu = { git = "https://github.com/gfx-rs/wgpu", features = ["spirv", "vulkan-portability"] }
 winit = { version = "0.26" }
 structopt = "0.3"
 strum = { version = "0.23.0", default_features = false, features = ["std", "derive"] }