Godot version:
3.2.2.stable

OS/device including version:
Windows10

Issue description:
CSG subtraction of a SphereMesh from a CubeMesh given in the attached scene is missing a triangle.

The crack does not go away when I turn off backface culling, so I think the triangle is being discarded rather than inverted. The too-close points should instead be pulled closed.

Steps to reproduce:
Cubemesh size = Vector3( 0.1, 0.1, 0.2 )
Spheremesh size = radius 0.1, height 0.2, radial_segments 12, rings 12 at position Vector3(-0.0544916, 0, -0.0286622 )

Minimal reproduction project:
csgmesh_cracked.zip