Qt: Refactor render widget state transitions

Recreate widget each time. Fixes fullscreen mode switches on D3D11 and
hopefully Wayland.

src/duckstation-qt/openglhostdisplay.h

@@ -0,0 +1,70 @@
+#pragma once
+
+// GLAD has to come first so that Qt doesn't pull in the system GL headers, which are incompatible with glad.
+#include <glad.h>
+
+// Hack to prevent Apple's glext.h headers from getting included via qopengl.h, since we still want to use glad.
+#ifdef __APPLE__
+#define __glext_h_
+#endif
+
+#include "common/gl/program.h"
+#include "common/gl/texture.h"
+#include "core/host_display.h"
+#include "qtdisplaywidget.h"
+#include "qthostdisplay.h"
+#include <QtGui/QOpenGLContext>
+#include <memory>
+
+class QtHostInterface;
+
+class OpenGLHostDisplay final : public QtHostDisplay
+{
+public:
+  OpenGLHostDisplay(QtHostInterface* host_interface);
+  ~OpenGLHostDisplay();
+
+  QtDisplayWidget* createWidget(QWidget* parent) override;
+
+  bool hasDeviceContext() const override;
+  bool createDeviceContext(bool debug_device) override;
+  bool initializeDeviceContext(bool debug_device) override;
+  bool makeDeviceContextCurrent() override;
+  void moveContextToThread(QThread* new_thread) override;
+  void destroyDeviceContext() override;
+  bool createSurface() override;
+  void destroySurface();
+
+  RenderAPI GetRenderAPI() const override;
+  void* GetRenderDevice() const override;
+  void* GetRenderContext() const override;
+
+  std::unique_ptr<HostDisplayTexture> CreateTexture(u32 width, u32 height, const void* initial_data,
+                                                    u32 initial_data_stride, bool dynamic) override;
+  void UpdateTexture(HostDisplayTexture* texture, u32 x, u32 y, u32 width, u32 height, const void* texture_data,
+                     u32 texture_data_stride) override;
+  bool DownloadTexture(const void* texture_handle, u32 x, u32 y, u32 width, u32 height, void* out_data,
+                       u32 out_data_stride) override;
+
+  void SetVSync(bool enabled) override;
+
+  void Render() override;
+
+private:
+  const char* GetGLSLVersionString() const;
+  std::string GetGLSLVersionHeader() const;
+
+  bool createImGuiContext() override;
+  void destroyImGuiContext() override;
+  bool createDeviceResources() override;
+  void destroyDeviceResources() override;
+
+  void renderDisplay();
+
+  std::unique_ptr<QOpenGLContext> m_gl_context = nullptr;
+
+  GL::Program m_display_program;
+  GLuint m_display_vao = 0;
+  GLuint m_display_nearest_sampler = 0;
+  GLuint m_display_linear_sampler = 0;
+};