Blender Git Loki

Fix T64164: Resetting a curvemap could result in a mirrored curve

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"

Reviewers: brecht

Maniphest Tasks: T64164

Differential Revision: https://developer.blender.org/D4809