Default to dxc compiler because fxc is too slow

This does require dxil.dll and dxcompiler.dll to be available,
falling back to fxc if these cannot be found.
Can be overridden by setting the WGPU_DX12_COMPILER
environment variable to "fxc".

src/graphics/d3d12.rs

@@ -112,7 +112,12 @@ pub fn initialize_xr_instance(
 
     let instance_descriptor = &wgpu_hal::InstanceDescriptor {
         name: &app_info.name,
-        dx12_shader_compiler: wgpu::util::dx12_shader_compiler_from_env().unwrap_or_default(),
+        dx12_shader_compiler: wgpu::util::dx12_shader_compiler_from_env().unwrap_or(
+            wgpu::Dx12Compiler::Dxc {
+                dxil_path: None,
+                dxc_path: None,
+            },
+        ),
         flags: wgpu::InstanceFlags::from_build_config().with_env(),
         gles_minor_version: Default::default(),
     };