UI: Add 1920x1080/1280x720 to the settings base res list

Adds 1920x1080 and 1280x720 to the drop-down list of "default"
base/canvas resolutions rather than just the user's monitor resolutions,
in case the user wants to set the canvas to 1920x1080 or 1280x720
without having to manually type it in.  Added as a minor convenience.

UI/window-basic-settings.cpp

@@ -1217,12 +1217,21 @@ void OBSBasicSettings::LoadResolutionLists()
 
 	ui->baseResolution->clear();
 
+	auto addRes = [this] (int cx, int cy)
+	{
+		QString res = ResString(cx, cy).c_str();
+		if (ui->baseResolution->findText(res) == -1)
+			ui->baseResolution->addItem(res);
+	};
+
 	for (QScreen* screen: QGuiApplication::screens()) {
 		QSize as = screen->size();
-		string res = ResString(as.width(), as.height());
-		ui->baseResolution->addItem(res.c_str());
+		addRes(as.width(), as.height());
 	}
 
+	addRes(1920, 1080);
+	addRes(1280, 720);
+
 	string outputResString = ResString(out_cx, out_cy);
 
 	ui->baseResolution->lineEdit()->setText(ResString(cx, cy).c_str());