This adds a new Add brush for the new curves object type in sculpt mode.
The brush is used to insert new curves (typically hair) on the surface object.
Supported features:
* Add single curve exactly at the cursor position when `Add Amount` is 1.
* Front faces only.
* Independent interpolate shape and interpolate length settings.
* Smooth and flat shading affects curve shape interpolation.
* Spherical and projection brush.
This also adds the `surface_triangle_index` and `surface_triangle_coordinate`
attributes. Those store information about what position on the surface each
added curve is attached to:
* `surface_triangle_index` (`int`): Index of the internal triangle that a curve
is attached to. `-1` when the curve is not attached to the surface.
* `surface_triangle_coordinate` (`float2`): First two numbers of a barycentric
coordinate that reference a specific position within the triangle.
Ref T96444.
Differential Revision: https://developer.blender.org/D14340
a58be397e2