This enables a union boolean mode for the trimming gestures tools which
adds geometry to the mesh instead of cutting it.
It also adds a Join mode, which adds the geometry directly without
using a boolean operation.
Depending if you plan to use dyntopo or not, it is useful to have both
options available.
This is using the full depth of the object from the camera view for the
depth of the geometry, but options for controlling the trimming depth in
all modes are going to be added in later patches
Reviewed By: mont29
Differential Revision: https://developer.blender.org/D9066
2b72860ff4