Broken by 4c9c9af2d6.
When Grease Pencil objects get converted (<= 4.2 to >= 4.3)
the code copies the legacy color over to the new one.
The issue is that the new color is a `float[3]` not a
`float[4]`! Thus the `flag` after the `color` was getting
overridden.
This fixes the issue by replacing the `copy_v4_v4` with a
`copy_v3_v3`.
Pull Request: https://projects.blender.org/blender/blender/pulls/137161
58fa6a50ce