Grease pencil build modifier additive mode has two problems that is
causing crashes and wrong build result when drawing in additive mode:
- Not assigning `dst_to_src_curve` for previously built curves.
- The way `previous_drawing` was acquired was wrong, this gives a zero
stroke count for additive build mode, which prevented the crash while
not in drawing mode (not on key frame), but is in fact incorrect for
the algorithm.
This PR handles these problems and additive mode now works correctly
and without crashes while drawing new frames.
Pull Request: https://projects.blender.org/blender/blender/pulls/130952
00f61f0913