Set ScreenPanelGL's minimum size immediately

Fixes GL context creation for OpenGL display on macOS using the wrong
size as the underlying window was not resized to the correct size by Qt
yet.

src/frontend/qt_sdl/main.cpp

@@ -1254,6 +1254,7 @@ ScreenPanelGL::ScreenPanelGL(QWidget* parent) : QWidget(parent), ScreenHandler(t
     setAttribute(Qt::WA_PaintOnScreen, true);
     setAttribute(Qt::WA_KeyCompression, false);
     setFocusPolicy(Qt::StrongFocus);
+    setMinimumSize(screenGetMinSize());
 }
 
 ScreenPanelGL::~ScreenPanelGL()