Sculpt: Add Gravity factor to brushes

A very useful old patch that never landed...

This adds a property to brushes to control how much of global gravity is applied to the stroke. This allows tweaking the behaviour of the global gravity per brush, and use brushes with and without gravity without changing the option each time the brush changes.

This is useful when working with cloth brush presets that needs gravity etc...

👉 archive.blender.org/developer/D9619