Verify that applications using the OpenGL graphics system on Symbian consume zero graphics memory when they are not visible

The Symbian port of Qt listens to the window server for events of type EEventWindowVisibilityChanged to know when a particular window has been completely obscured. When this happens, Qt will deallocate the resources for this window by deleting the backing store (and thereby window surface).

It should be verified that the OpenGL graphics systems does reference counting of the EGL contexts and when there are no more contexts available, the texture cache is cleared to save graphics memory.

It is a known issue that QGLPixmapData does not free it's resources when all EGL contexts are destroyed, so this should also be considered when implementing QTBUG-252 such that background applications consume zero graphics memory.