Blender Git Loki

Implement basic global tablet pressure curve options.

Grease Pencil already implements support for full-featured
per-brush pressure curves, but it is useful to have some
basic global settings that affect all brushes and tools.

This adds two simple options:

- Raw pressure required to achieve full brush intensity.
- Softness control, using a gamma curve internally.

The most important one is the max pressure setting, because it is
critical for ergonomics, but the Linux Wacom driver lacks it.

The softness option internally converts to gamma = 4^-softness.

Reviewers: brecht, campbellbarton

Differential Revision: https://developer.blender.org/D3967