Remove Platform::sceneEglConfig()

With per-screen rendering, every output may have different EGLConfig.
Having a single global EGLConfig doesn't work out well.

This change removes Platform::sceneEglConfig(). It's used primarily to
create the global share context. In hindsight, the global share context
can be created without EGLConfig as it's never made current.

EGL_NO_CONFIG_KHR is part of EGL_KHR_no_config_context extension, which
is supported by both Mesa and NVIDIA driver so it should be safe to make
it mandatory.

src/platform.h

@@ -119,20 +119,6 @@ public:
      */
     void setSceneEglGlobalShareContext(EGLContext context);
 
-    /**
-     * The EglConfig used by the compositing scene.
-     */
-    EGLConfig sceneEglConfig() const {
-        return m_eglConfig;
-    }
-    /**
-     * Sets the @p config used by the compositing scene.
-     * @see sceneEglConfig
-     */
-    void setSceneEglConfig(EGLConfig config) {
-        m_eglConfig = config;
-    }
-
     /**
      * Implementing subclasses should provide a size in case the backend represents
      * a basic screen and uses the BasicScreens.
@@ -547,7 +533,6 @@ private:
     int m_initialOutputCount = 1;
     qreal m_initialOutputScale = 1;
     EGLDisplay m_eglDisplay;
-    EGLConfig m_eglConfig = nullptr;
     EGLContext m_context = EGL_NO_CONTEXT;
     EGLContext m_globalShareContext = EGL_NO_CONTEXT;
     int m_hideCursorCounter = 0;

src/platformsupport/scenes/opengl/abstract_egl_backend.cpp

@@ -40,7 +40,6 @@ static bool isOpenGLES_helper()
 static EGLContext ensureGlobalShareContext()
 {
     const EGLDisplay eglDisplay = kwinApp()->platform()->sceneEglDisplay();
-    const EGLConfig eglConfig = kwinApp()->platform()->sceneEglConfig();
 
     if (kwinApp()->platform()->sceneEglGlobalShareContext() != EGL_NO_CONTEXT) {
         return kwinApp()->platform()->sceneEglGlobalShareContext();
@@ -56,7 +55,7 @@ static EGLContext ensureGlobalShareContext()
         attribs = builder.build();
     }
 
-    s_globalShareContext = eglCreateContext(eglDisplay, eglConfig, EGL_NO_CONTEXT, attribs.data());
+    s_globalShareContext = eglCreateContext(eglDisplay, EGL_NO_CONFIG_KHR, EGL_NO_CONTEXT, attribs.data());
     if (s_globalShareContext == EGL_NO_CONTEXT) {
         qCWarning(KWIN_OPENGL, "Failed to create global share context: 0x%x", eglGetError());
     }
@@ -108,7 +107,6 @@ void AbstractEglBackend::cleanup()
         eglDestroyContext(m_display, m_context);
         eglReleaseThread();
         kwinApp()->platform()->setSceneEglContext(EGL_NO_CONTEXT);
-        kwinApp()->platform()->setSceneEglConfig(nullptr);
     }
 }
 
@@ -144,6 +142,17 @@ bool AbstractEglBackend::initEglAPI()
     qCDebug(KWIN_OPENGL) << "EGL version: " << major << "." << minor;
     const QByteArray eglExtensions = eglQueryString(m_display, EGL_EXTENSIONS);
     setExtensions(eglExtensions.split(' '));
+
+    const QByteArray requiredExtensions[] = {
+        QByteArrayLiteral("EGL_KHR_no_config_context"),
+    };
+    for (const QByteArray &extensionName : requiredExtensions) {
+        if (!hasExtension(extensionName)) {
+            qCWarning(KWIN_OPENGL) << extensionName << "extension is unsupported";
+            return false;
+        }
+    }
+
     setSupportsSurfacelessContext(hasExtension(QByteArrayLiteral("EGL_KHR_surfaceless_context")));
     setSupportsNativeFence(hasExtension(QByteArrayLiteral("EGL_ANDROID_native_fence_sync")));
     return true;
@@ -355,9 +364,6 @@ void AbstractEglBackend::setEglDisplay(const EGLDisplay &display) {
 void AbstractEglBackend::setConfig(const EGLConfig &config)
 {
     m_config = config;
-    if (isPrimary()) {
-        kwinApp()->platform()->setSceneEglConfig(config);
-    }
 }
 
 void AbstractEglBackend::setSurface(const EGLSurface &surface)