Moving a set of keyframes could cause crashes by setting invalid
`drawing_index` in `GreasePencilFrame` data.
The transform operator for grease pencil keyframes can add and remove
keyframes by overwriting existing frames. The `move_duplicate_frames`
function in particular has to keep track of drawing user counts to
ensure that the drawings referenced by the frames are still alive at the
end.
This was broken when moving multiple keyframes at once, such that later
keyframes would overwrite the target positions for earlier frames (for
example moving frames [1,2,3] to [2,3,4]). The `move_duplicate_frames`
was first removing the source frame and then adding it back at the
destination. In case the source frame was already the destination of an
earlier keyframe, this will cause incorrect user count because the frame
being removed is not the same as the one being added back.
To avoid this problem, remove all the source keyframes first before any
other modification of the destination layer. That way we can be sure the
frames at the source index is actually the expected frame.
Pull Request: https://projects.blender.org/blender/blender/pulls/119207
f04bf1694f