A template_curve_mapping with brush=True was always resetting the curve
to a positive slope (descending from left to right).
Behaviour is now changed so that specifying "use_negative_slope" is
taken into account as well.
Also use this for:
- paint "cavity_curve"
- gpencil "curve_sensitivity" / "curve_strength" / "curve_jitter" /
"interpolation_curve"