Image lifecycle and management

[namse]

Current namui system always pushs images in memory and never frees them.

If image is not more useful, it should be dropped.

For example, Video's frame should be dropped when it already displayed.
Or the image asset of scene should be dropped when the scene had been changed.

Most of case, RLU cache would fixes this problem.
But there would be a need that user want to handle the timing of image drop.

I need policy to manage image.