UI: Add warning if using no/multiple tracks

Certain output formats don't support multiple tracks, so it's important
to warn the user if they select multiple tracks.

Also warn the user if they select no tracks.

obs/window-basic-settings.cpp

@@ -379,6 +379,17 @@ OBSBasicSettings::OBSBasicSettings(QWidget *parent)
 			"hotkey_unregister", ReloadHotkeysIgnore, this);
 
 	LoadSettings(false);
+
+	// Add warning checks to advanced output recording section controls
+	connect(ui->advOutRecTrack1, SIGNAL(clicked()),
+			this, SLOT(AdvOutRecCheckWarnings()));
+	connect(ui->advOutRecTrack2, SIGNAL(clicked()),
+			this, SLOT(AdvOutRecCheckWarnings()));
+	connect(ui->advOutRecTrack3, SIGNAL(clicked()),
+			this, SLOT(AdvOutRecCheckWarnings()));
+	connect(ui->advOutRecTrack4, SIGNAL(clicked()),
+			this, SLOT(AdvOutRecCheckWarnings()));
+	AdvOutRecCheckWarnings();
 }
 
 void OBSBasicSettings::SaveCombo(QComboBox *widget, const char *section,
@@ -2533,3 +2544,40 @@ void OBSBasicSettings::AdvancedChanged()
 		EnableApplyButton(true);
 	}
 }
+
+void OBSBasicSettings::AdvOutRecCheckWarnings()
+{
+	auto Checked = [](QCheckBox *box)
+	{
+		return box->isChecked() ? 1 : 0;
+	};
+
+	QString msg;
+	uint32_t tracks =
+		Checked(ui->advOutRecTrack1) +
+		Checked(ui->advOutRecTrack2) +
+		Checked(ui->advOutRecTrack3) +
+		Checked(ui->advOutRecTrack4);
+	const char *objectName = nullptr;
+
+	if (tracks == 0) {
+		msg = QTStr("OutputWarnings.NoTracksSelected");
+		objectName = "errorLabel";
+
+	} else if (tracks > 1) {
+		msg = QTStr("OutputWarnings.MultiTrackRecording");
+		objectName = "warningLabel";
+	}
+
+	delete advOutRecWarning;
+
+	if (!msg.isEmpty()) {
+		advOutRecWarning = new QLabel(msg, this);
+		advOutRecWarning->setObjectName(objectName);
+
+		QFormLayout *formLayout = reinterpret_cast<QFormLayout*>(
+				ui->advOutRecTopContainer->layout());
+
+		formLayout->addRow(nullptr, advOutRecWarning);
+	}
+}