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 of the
`StrokeCache` is only updated at the beginning of the stroke. It 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.
Additionally, the operator level `pen_flip` property is moved to the
generic paint stroke operator instead of being defined as a property of
the `OperatorStrokeElement` struct. This value is now only calculated
at the beginning of the stroke instead of before each stroke step.
Pull Request: https://projects.blender.org/blender/blender/pulls/129559
6df437be5f