Blender Git Loki

GPencil Fix: Added checks to ensure that copy/paste doesn't paste incompatible strokes

There was a problem with the copy/paste functionality, where it would be possible to
paste 3d strokes into 2D editors, or 2D strokes into the 3D view. The problem with
that though is that these will not show up, and because there's no feedback at the
time, users may end up doing this pasting several times.