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Fix T72092: Clay Strips Brush stroke crash with Brush Detail Size

Some other areas in the brush code outside sculpt mode assume that
pressure is multiplied directly on top of the initial size. This patch
calculates the pixel radius correctly using the brush size from sculpt
mode to get the dyntopo detail size.
When the new brush input system is in place, all these values will come
directly from the brush input code with all the custom curves applied
per brush, so all paint modes will have a correct brush behavior and all
this sculpt mode specific code won't be necessary.

Reviewed By: jbakker

Maniphest Tasks: T72092

Differential Revision: https://developer.blender.org/D6339