Blender Git Loki

Fix metadata display in sequencer lost after doing a preprocess
transform.

IMB_makeSingleUser makes a copy and destroys metadata. I am not sure if
this is the safest way to make a single user ImBuf (setting the refcount
to 0 is simpler and there's less, but no zero, risk of dangling
pointers) but I will leave this as is for now in case there is an actual
need for a copy here. The alternative approach should be tested at some
point.