Silence downlevel caps warning on secondary backends

Author: kvark

wgpu-core/src/instance.rs

@@ -288,7 +288,9 @@ impl<A: HalApi> Adapter<A> {
         }
 
         let caps = &self.raw.capabilities;
-        if !caps.downlevel.is_webgpu_compliant() {
+        if wgt::Backends::PRIMARY.contains(wgt::Backends::from(A::VARIANT))
+            && !caps.downlevel.is_webgpu_compliant()
+        {
             let missing_flags = wgt::DownlevelFlags::compliant() - caps.downlevel.flags;
             log::warn!(
                 "Missing downlevel flags: {:?}\n{}",