Add Metal support to Qt frontend and clean up renderer creation code (#795)

* Qt: Initial support for Metal renderer

* Clean up graphics context code

* Nits

* More nits

* Qt: Move screen-related stuff to own folder

* Qt: Make screen widget polymorphic

* Qt: Re-add Metal

* Add factory for screen widget

* Qt: Support compilation without Metal

* Qt: Fix build without Metal

* Oops

* oops

include/panda_qt/screen/screen_gl.hpp

@@ -0,0 +1,18 @@
+#pragma once
+#include <memory>
+
+#include "gl/context.h"
+#include "panda_qt/screen/screen.hpp"
+
+class ScreenWidgetGL : public ScreenWidget {
+	std::unique_ptr<GL::Context> glContext = nullptr;
+
+  public:
+	ScreenWidgetGL(API api, ResizeCallback resizeCallback, QWidget* parent = nullptr);
+
+	virtual GL::Context* getGLContext() override;
+	virtual bool createContext() override;
+
+	virtual void resizeDisplay() override;
+	virtual void resizeSurface(u32 width, u32 height) override;
+};