This commit builds off of 5f8311f596 to support collapsing the custom
paint curve to reduce the amount of information shown to the user.
To support this, a new field is added to the `Paint` struct for this
visibility flag. This value is stored on a per-mode level, not a
per-brush level, as a user editing this curve is likely to not need
the visibility granularity on a per-brush basis.
Additionally, changes are included here to draw the curve button
itself as inactive if the parent layout it is part of is inactive.
Notably, this does not make the curve read only - it only changes
the appearance.
Pull Request: https://projects.blender.org/blender/blender/pulls/145699
5b02341a11