Blender Git Loki

GPencil Copy/Paste Fix: Copying/Pasting strokes between datablocks would crash

The problem was that newly pasted strokes were still using colours from
the original datablock. As a result, you'd either get an immediate crash,
or if you managed to save the file before it crashed, each stroke would get
reloaded with a dummy colour.

This commit fixes makes it possible to copy/paste strokes between datablocks
again. However, there are still problems when trying to paste across file
boundaries (i.e. copy strokes in one file, paste in another), which the next
commit will address.