Introduced in f4e670af2c
The `use_negative_slope` parameter was effectively ignored when `brush`
was also passed in as a parameter, always defaulting to a positive
slope (ascending from left to right).
Additionally, the `use_negative_slope` property was incorrectly
specified for many properties: In general, most brush properties have
positive slope as they correspond to a pressure value being modulated.
This commit fixes the behavior and updates the corresponding
properties so they continue to work.
Pull Request: https://projects.blender.org/blender/blender/pulls/145823
635f42c023