This adds a function `GreasePencil::count_frame_users_for_drawings()`
that computes the drawing user counts from the layer frames. This is used
to correctly initialize the runtime drawing user counts when reading the ID.
Note that we don't expose any functionality to instance drawings currently.
This change is also in preparation for when this will be possible.
Also adds a function `GreasePencil::validate_drawing_user_counts()` that
compares the actual user counts with the runtime user counts that are
stored. Only runs these checks in debug builds.
Pull Request: https://projects.blender.org/blender/blender/pulls/141458
45ab790e80