DFAO in 4.9 has significant artifacts on some machines

Reported in this thread
https://forums.unrealengine.com/showthread.php?78320-4-9-Horrible-DFAO-artifacts

This looks to be a machine specific issue. I'd like to get some testing on different video cards and see if we can track down which ones don't work correctly. The implementation of DFAO has changed significantly between 4.8 and 4.9 and if I know what video cards the new version doesn't work on, I might be able to fix it in time for 4.9 and avoid a big regression.

1. Launch QAGame
2. Check 'Generate Mesh Distance Fields' in Renderer project settings
3. Exit editor, start editor
4. Create new level
5. Delete directional light and place skylight
6. Change to movable
7. Duplicate static meshes and place new ones to match the scene in the thread.