UI: Fix linux display/resize bug with preview widget

On linux, the main window isn't immediately set to a native widget, so
the call:

connect(windowHandle(), &QWindow::screenChanged, displayResize);

on line 189 will fail, causing the preview widget to not properly resize
when the main window is resized.

Closes jp9000/obs-studio#776

UI/window-basic-main.cpp

@@ -123,6 +123,8 @@ OBSBasic::OBSBasic(QWidget *parent)
 	: OBSMainWindow  (parent),
 	  ui             (new Ui::OBSBasic)
 {
+	setAttribute(Qt::WA_NativeWindow);
+
 	projectorArray.resize(10, "");
 	previewProjectorArray.resize(10, 0);