QPdf: replace a hand-written qWarning with Q_UNIMPLEMENTED

But keep the 'Implement me' as a comment.

Remove default label from the switch to indicate that
the warning is only for the conical gradient case,
and not for the NoGradient case. Also enables
compiler warnings should we ever decide to add
another gradient type.

Change-Id: I891e652ff0293b4b4c254c7a55ebc6120b4fbcb6
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/gui/painting/qpdf.cpp

@@ -2215,8 +2215,10 @@ int QPdfEnginePrivate::generateGradientShader(const QGradient *gradient, const Q
     case QGradient::RadialGradient:
         return generateRadialGradientShader(static_cast<const QRadialGradient *>(gradient), matrix, alpha);
     case QGradient::ConicalGradient:
-    default:
-        qWarning("Implement me!");
+        Q_UNIMPLEMENTED(); // ### Implement me!
+        break;
+    case QGradient::NoGradient:
+        break;
     }
     return 0;
 }