Add support for multiple character images to improve rendering of teeth

My initial experiments suggest that when supplying photo character images the most realistic lip-syncing results come if the character has their mouth in a neutral, closed position. However, this leads to a problem in that then the model doesn’t have any knowledge of the appearance of the character’s teeth or mouth interior and so it essentially has to guess, leading to unrealistic results.

My proposal is that you should make it possible to supply additional reference images for a character, which should then be taken into account by the model when creating the final video sequence. Users could be encouraged to supply at least one photo of the character with their teeth visible. Overall, this should lead to more realistic results.