metal: Set preserveInvariance for shader options

This fixes, for example, depth prepass resulting in ever so slightly
different depth values for later render passes.

wgpu-hal/src/metal/device.rs

@@ -75,6 +75,7 @@ impl super::Device {
 
         let options = mtl::CompileOptions::new();
         options.set_language_version(self.shared.private_caps.msl_version);
+        options.set_preserve_invariance(true);
 
         let library = self
             .shared