Prevent moving around Sprites via std::vector operations

This removes the need for the keep_graphics flag.
Remove Sprite copying from GameStateCutscene

This changes image loading a bit. Instead of loading images when
creating the game state, images are loaded when creating each scene.
Don't copy Sprite of NPC portrait to MenuTalker

Usea pointer instead. This removes the need for keep_graphics in the
Sprite class.
Remove keep_graphics flag from Sprite class
Merge pull request #938 from dorkster/keep_graphics_removal

Remove keep_graphics flag from Sprite class