Blender Git Loki

Fix for [#25713] VSE shows and renders wrong straight alpha gradient even after convert to premul is checked
* Caching of the start and end stills were just referencing the original imbuf (which got premultiplied after the caching), so as a result most of the time the premul was applied twice.
* Now the start and end stills are stored in the cache as duplicates of the original (non modified) imbuf.