Due to the refactoring of the overlay engine the draw caches were
changed. The sphere batch used to have positions and normals. After the
refactoring it didn't had the normals anymore. The normals are needed
for shading. As they were not there the look dev spheres were rendered
black.
This change add the `nor` attribute to `DRW_cache_sphere_get` batch.
Reviewed By: fclem
Differential Revision: https://developer.blender.org/D6393
b30a722f2d