Log Message:
Changes 4.6.1:
* Optimized empty QUrl creation
* Improve performance of getting the canonical filename on Linux
  and Symbian by using realpath() system call.
* Avoid stat() when opening a file.
* Do not look at the Qt patch-level version embedded in plugins'
  buildkeys when trying to determine if the plugin is compatible
* Bug-fixes
* load() and loadFromData() can now support compressed GL textures
  in the DDS, ETC1, PVRTC2, and PVRTC4 formats if the OpenGL graphics
  system is active and the appropriate extensions are present in the
  GL implementation.
* Fixed a small leak when using the new QPixmapCache::Key based API.