Prior to this commit, the brush action was determined on a step by step
basis by peeking at the `StrokeCache` `bstrength` property and
determining which brush to use based on if the value was negative or
not. The sign of this value, however, was not static across the
entirety of a brush stroke, as it is calculated from three separate
fields, one of which could vary over the course of a stroke when using a
tablet, the `pen_flip` property.
To fix this issue, this commit ensures the `pen_flip` field is only
updated at the beginning of the stroke and also adds a new boolean to
store the initial direction of the stroke to reduce further ambiguity
when comparing the sign of the brush strength.
Pull Request: https://projects.blender.org/blender/blender/pulls/129184
304800db73