Prevent DrawTextItemDevice::metric warning on Mac

Implement PdmDevicePixelRatio.

Task-number: QTBUG-36419
Change-Id: I4d2822d01e08c80e9b829d5f524e8e761e694fba
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/gui/text/qstatictext.cpp

@@ -573,6 +573,9 @@ namespace {
             case PdmDepth:
                 val = 24;
                 break;
+            case PdmDevicePixelRatio:
+                val = 1;
+                break;
             default:
                 val = 0;
                 qWarning("DrawTextItemDevice::metric: Invalid metric command");