System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Blender Version
Broken: version: 3.0.0 Alpha, branch: master, commit date: 2021-06-21 16:25, hash: rBb11a463e4fcd
Worked: Assert was introduced by rB819152527fd4, so any prior commit should be fine
**Short description of error**
Decimating a non-planar face causes an assertion failure.
**Exact steps for others to reproduce the error**
1. Delete the starting scene
2. Create a Plane
3. Go into edit mode
4. Move one of the vertices so that the mesh isn't planar
5. Decimate the face
6. Assertion failure
Obviously, this requires a debug build.
Failing assertion:
```
#ifdef DEBUG
/* The face normal is used for projecting faces into 2D space for tessellation.
* Invalid normals may result in invalid tessellation.
* Either `face_normal` should be true or normals should be updated first. */
BLI_assert(face_normal || BM_face_is_normal_valid(efa));
#endif
```