I cleaned up the cube brush tip code quite a bit; more remains
to be done. There is a new function to initialize cube
tip matrices, SCULPT_cube_tip_init. It's currently only
used by the paint brush, I'll need to do a bit of testing
before using it for clay strips and multiplane scrape.
Note: SCULPT_cube_tip_init uses the brush local matrix code
to avoid code duplication (and to take advantage of the debouncing
that is done there).
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