Initialize the supports partial update flag to false

With 870679e46f, if the partial update
extension is unsupported, setSupportsPartialUpdate() won't be called.

The problem is that it may leave OpenGLBackend::supportsPartialUpdate()
uninitialized, which can lead to a crash if an OpenGL render backend
tries to call eglSetDamageRegionKHR() and EGL_KHR_partial_update is
unsupported.

platformsupport/scenes/opengl/backend.h

@@ -295,7 +295,7 @@ private:
     /**
      * @brief Whether the backend supports EGL_KHR_partial_update
      */
-    bool m_havePartialUpdate;
+    bool m_havePartialUpdate = false;
     bool m_haveSwapBuffersWithDamage = false;
     /**
      * @brief Whether the backend supports EGL_KHR_surfaceless_context.