This adds a function to merge layers in original Grease Pencil geometry.
It also adds an operator to merge layers as well as some tests for the `merge_layers` function.
The operator has 3 modes:
* `Merge Down`: Combine the active layer with the layer just below (if there is one).
* `Merge Group`: Combine all the layers in a group into one single layer and remove
the group. Can be accessed in the right-click menu of groups.
* `Merge All`: Combine all the layers of the object into one single layer.
All of these can be accessed in the `Extras` menu next to the layer tree.
Pull Request: https://projects.blender.org/blender/blender/pulls/128201
1829982d3b