Preserve credential provider logo transparency

src/SGU.CredentialProvider/SguCredentialTile.cs

@@ -5,7 +5,11 @@ using SGU.AuthBroker.Core.Identity;
 
 namespace SGU.CredentialProvider;
 
-internal sealed class SguCredentialTile : CredentialTile2
+// LogonUI can only preserve the alpha channel of the provider logo when the
+// credential exposes ICredentialProviderCredential3. CredentialTile2 returns
+// an HBITMAP, which composites transparent pixels against #464646 and produces
+// a visible grey square around the circular SGU mark.
+internal sealed class SguCredentialTile : CredentialTile3
 {
     private TextboxControl userNameControl = null!;
     private SecurePasswordTextboxControl passwordControl = null!;