plugins/qpa: use a surfaceless context for offscreen surfaces

According to Qt documentation, offscreen surfaces aren't actually for rendering,
so using a surffaceless context is ok

src/plugins/qpa/eglplatformcontext.cpp

@@ -58,18 +58,9 @@ EGLPlatformContext::~EGLPlatformContext()
     }
 }
 
-static EGLSurface eglSurfaceForPlatformSurface(QPlatformSurface *surface)
-{
-    if (surface->surface()->surfaceClass() == QSurface::Window) {
-        return EGL_NO_SURFACE; // EGL_KHR_surfaceless_context is required
-    } else {
-        return static_cast<OffscreenSurface *>(surface)->eglSurface();
-    }
-}
-
 bool EGLPlatformContext::makeCurrent(QPlatformSurface *surface)
 {
-    const bool ok = m_eglContext->makeCurrent(eglSurfaceForPlatformSurface(surface));
+    const bool ok = m_eglContext->makeCurrent();
     if (!ok) {
         qCWarning(KWIN_QPA, "eglMakeCurrent failed: %x", eglGetError());
         return false;

src/plugins/qpa/offscreensurface.cpp

@@ -22,34 +22,8 @@ namespace QPA
 
 OffscreenSurface::OffscreenSurface(QOffscreenSurface *surface)
     : QPlatformOffscreenSurface(surface)
-    , m_eglDisplay(kwinApp()->outputBackend()->sceneEglDisplayObject())
+    , m_format(surface->requestedFormat())
 {
-    const QSize size = surface->size();
-
-    EGLConfig config = configFromFormat(m_eglDisplay, surface->requestedFormat(), EGL_PBUFFER_BIT);
-    if (config == EGL_NO_CONFIG_KHR) {
-        return;
-    }
-
-    const EGLint attributes[] = {
-        EGL_WIDTH, size.width(),
-        EGL_HEIGHT, size.height(),
-        EGL_NONE};
-
-    m_surface = eglCreatePbufferSurface(m_eglDisplay, config, attributes);
-    if (m_surface == EGL_NO_SURFACE) {
-        return;
-    }
-
-    // Requested and actual surface format might be different.
-    m_format = formatFromConfig(m_eglDisplay, config);
-}
-
-OffscreenSurface::~OffscreenSurface()
-{
-    if (m_surface != EGL_NO_SURFACE) {
-        eglDestroySurface(m_eglDisplay, m_surface);
-    }
 }
 
 QSurfaceFormat OffscreenSurface::format() const
@@ -59,12 +33,7 @@ QSurfaceFormat OffscreenSurface::format() const
 
 bool OffscreenSurface::isValid() const
 {
-    return m_surface != EGL_NO_SURFACE;
-}
-
-EGLSurface OffscreenSurface::eglSurface() const
-{
-    return m_surface;
+    return true;
 }
 
 } // namespace QPA

src/plugins/qpa/offscreensurface.h

@@ -25,18 +25,12 @@ class OffscreenSurface : public QPlatformOffscreenSurface
 {
 public:
     explicit OffscreenSurface(QOffscreenSurface *surface);
-    ~OffscreenSurface() override;
 
     QSurfaceFormat format() const override;
     bool isValid() const override;
 
-    EGLSurface eglSurface() const;
-
 private:
     QSurfaceFormat m_format;
-
-    EglDisplay *m_eglDisplay = nullptr;
-    EGLSurface m_surface = EGL_NO_SURFACE;
 };
 
 } // namespace QPA