UI: Add OBSBasicPreview::Get helper func

UI/window-basic-preview.cpp

@@ -1587,3 +1587,8 @@ void OBSBasicPreview::SetScalingAmount(float newScalingAmountVal) {
 	scrollingOffset.y *= newScalingAmountVal / scalingAmount;
 	scalingAmount = newScalingAmountVal;
 }
+
+OBSBasicPreview *OBSBasicPreview::Get()
+{
+	return OBSBasic::Get()->ui->preview;
+}

UI/window-basic-preview.hpp

@@ -91,6 +91,8 @@ public:
 	OBSBasicPreview(QWidget *parent, Qt::WindowFlags flags = 0);
 	~OBSBasicPreview();
 
+	static OBSBasicPreview *Get();
+
 	virtual void keyPressEvent(QKeyEvent *event) override;
 	virtual void keyReleaseEvent(QKeyEvent *event) override;