This is part of #121565.
Refactors `legacy_gpencil_to_grease_pencil` to use
`GreasePencil::insert_frame` instead of manually creating
the drawing array.
Now `legacy_gpencil_frame_to_grease_pencil_drawing`
actually returns a drawing by value.
Note: This means that we're now resizing the drawing
array for every frame in every layer, but this can be optimized
later.
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