qpa: Handle compositor restarts in EGLPlatformContext

If the compositing has been restarted, the global share context will be destroyed and we no longer can receive framebuffer objects from internal windows. From user's perspective, it can be seen as tabbox not working anymore.

We need to mark QOpenGLContext objects as invalid so QtQuick machinery can re-create all OpenGL related resources, e.g. textures, etc.

BUG: 415798 FIXED-IN: 5.20.1