Lazily load images to avoid out of memory

[JasY95]

A useful feature would be to choose whether to load the images into memory, for particularly large EXR files. Loading a sequence of 600 130MB caused the system to entirely lock up.

[Tom94]

Agreed! Unfortunately, this requires a few non-trivial changes throughout tev, so I can't say when I'll get to it, but I'm putting it high on the todo list.