This PR implements the "Smooth" operator in Weight Paint mode for GPv3.
The Smooth operator smooths the weights in the active vertex group by
applying a gaussian blur to the vertex weights.
The operator has two properties:
- `factor` (0.0 - 1.0): The extent to which the smoothed weight is
applied to the original weight, where 0.0 is 'keep the original weight'
and 1.0 is 'replace fully with the smoothed weight'.
- `repeat`: The number of times the smoothing is executed. A higher
value means more smoothing.
The operator is added to the 'Weight' menu in Weight Paint mode.
Pull Request: https://projects.blender.org/blender/blender/pulls/126429
9c9b562d54