Add a primary backend that can be set

Add a primary backend that can be set

wgpu/src/settings.rs

@@ -78,6 +78,7 @@ fn backend_from_env() -> Option<wgpu::BackendBit> {
             "dx11" => wgpu::BackendBit::DX11,
             "gl" => wgpu::BackendBit::GL,
             "webgpu" => wgpu::BackendBit::BROWSER_WEBGPU,
+            "primary" => wgpu::BackendBit::PRIMARY,
             other => panic!("Unknown backend: {}", other),
         }
     })