Cache generated Lanczos textures.

This provides a significant performance improvement especially for Present Windows.
Whenever a lanczos resampling is done the resulted texture is cached, till the window is damaged or a texture of different size is required.

This means that for a taskbar thumbnail the resampling only occurs once unless the window is damaged and for present windows it only occurs when the windows are resized and then only after the windows have already been moved. Highlighting windows does not cause a resampling any more.

See review request http://svn.reviewboard.kde.org/r/5708/