Expose/fake native display refresh rate. Only has a real value on Android.

base/PCMain.cpp

@@ -316,6 +316,8 @@ int System_GetPropertyInt(SystemProperty prop) {
 	switch (prop) {
 	case SYSPROP_AUDIO_SAMPLE_RATE:
 		return 44100;
+	case SYSPROP_DISPLAY_REFRESH_RATE:
+		return 60000;
 	default:
 		return -1;
 	}