Blender Git Loki

Fix: Pasting GP strokes files between files (or when the original colors were deleted) would crash

The problem was that the strokes in the copy-paste buffer could be keeping
dangling pointers to colors that were already freed. Therefore, this commit
makes it so that when copying the strokes, we now make copies of the colors
and put them in a hashtable beside the stroke buffer. This is convenient,
as it saves us having to look up what colours need to be copied over each
time when pasting.