If strip is split or duplicated, thumbnails are rendered twice, even
though source file is same for both strips. This is mainly due to cache
design.

This patch allows for sharing of images between strips.

Quick testing of implementation revealed some issues:

• Normally thumbnails out of view were freed, once too many thumbnails are cached. There is no good way of checking to what frame this strip belongs to if it is shared between strips however. Not sure how to resolve this problem.
• This method is using string for hashing, but frame number needs to be appended, which is not nice.
• Adding support for strips that reference other data like movieclip or scene could be problematic, but at least it will increase complexity.

Otherwise it works relatively well.

ref T91618