[desktop] More qt_common reorganization (#3916)

Ported from QML branch.

Main "big" change is that EmuThread is now a shared state in QtCommon,
not individually managed/passed around by GRenderWindow and MainWindow.

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3916
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

src/qt_common/render/context.h

@@ -0,0 +1,107 @@
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#pragma once
+
+#include <QSurface>
+#include "common/logging.h"
+#include "common/settings.h"
+#include "core/frontend/graphics_context.h"
+
+#ifdef HAS_OPENGL
+#include <QOffscreenSurface>
+#include <QOpenGLContext>
+#endif
+
+#ifdef HAS_OPENGL
+class OpenGLSharedContext : public Core::Frontend::GraphicsContext {
+public:
+    /// Create the original context that should be shared from
+    explicit OpenGLSharedContext(QSurface* surface_) : surface{surface_} {
+        QSurfaceFormat format;
+        format.setVersion(4, 6);
+        format.setProfile(QSurfaceFormat::CompatibilityProfile);
+        format.setOption(QSurfaceFormat::FormatOption::DeprecatedFunctions);
+        if (Settings::values.renderer_debug) {
+            format.setOption(QSurfaceFormat::FormatOption::DebugContext);
+        }
+        // TODO: expose a setting for buffer value (ie default/single/double/triple)
+        format.setSwapBehavior(QSurfaceFormat::DefaultSwapBehavior);
+        format.setSwapInterval(0);
+
+        context = std::make_unique<QOpenGLContext>();
+        context->setFormat(format);
+        if (!context->create()) {
+            LOG_ERROR(Frontend, "Unable to create main openGL context");
+        }
+    }
+
+    /// Create the shared contexts for rendering and presentation
+    explicit OpenGLSharedContext(QOpenGLContext* share_context, QSurface* main_surface = nullptr) {
+
+        // disable vsync for any shared contexts
+        auto format = share_context->format();
+        const int swap_interval =
+            Settings::values.vsync_mode.GetValue() == Settings::VSyncMode::Immediate ? 0 : 1;
+
+        format.setSwapInterval(main_surface ? swap_interval : 0);
+
+        context = std::make_unique<QOpenGLContext>();
+        context->setShareContext(share_context);
+        context->setFormat(format);
+        if (!context->create()) {
+            LOG_ERROR(Frontend, "Unable to create shared openGL context");
+        }
+
+        if (!main_surface) {
+            offscreen_surface = std::make_unique<QOffscreenSurface>(nullptr);
+            offscreen_surface->setFormat(format);
+            offscreen_surface->create();
+            surface = offscreen_surface.get();
+        } else {
+            surface = main_surface;
+        }
+    }
+
+    ~OpenGLSharedContext() {
+        DoneCurrent();
+    }
+
+    void SwapBuffers() override {
+        context->swapBuffers(surface);
+    }
+
+    void MakeCurrent() override {
+        // We can't track the current state of the underlying context in this wrapper class because
+        // Qt may make the underlying context not current for one reason or another. In particular,
+        // the WebBrowser uses GL, so it seems to conflict if we aren't careful.
+        // Instead of always just making the context current (which does not have any caching to
+        // check if the underlying context is already current) we can check for the current context
+        // in the thread local data by calling `currentContext()` and checking if its ours.
+        if (QOpenGLContext::currentContext() != context.get()) {
+            context->makeCurrent(surface);
+        }
+    }
+
+    void DoneCurrent() override {
+        context->doneCurrent();
+    }
+
+    QOpenGLContext* GetShareContext() {
+        return context.get();
+    }
+
+    const QOpenGLContext* GetShareContext() const {
+        return context.get();
+    }
+
+private:
+    // Avoid using Qt parent system here since we might move the QObjects to new threads
+    // As a note, this means we should avoid using slots/signals with the objects too
+    std::unique_ptr<QOpenGLContext> context;
+    std::unique_ptr<QOffscreenSurface> offscreen_surface{};
+    QSurface* surface;
+};
+#endif
+
+class DummyContext : public Core::Frontend::GraphicsContext {};