Prior to this commit, the `object.subdivision_set` would prevent
actually applying a subdivision to a multires modifier when the relative
option was set. This is commonly accessed via Alt-1 / Alt-2 or D / Shift
D in Sculpt mode.
However, when the multires modifier did not already exist, pressing
these keybinds would still create the modifier and further subdivide the
mesh.
To fix this, this commit adds a hidden property to the operator:
`ensure_modifier` to indicate if the keybind should create the modifier
or not.
Pull Request: https://projects.blender.org/blender/blender/pulls/130254
ee67c098d9