If a mirror object is used in a mirror modifier, sculptmode did not take
this into account (and instead always clipped on the sculpt objects
local axis).
Now take this into account by storing a matrix in the preparation
function `sculpt_init_mirror_clipping` and use that later in
`SCULPT_clip`.
Maniphest Tasks: T94564
Differential Revision: https://developer.blender.org/D13711
13e7065dd2