Entering play mode breaks lighting UVs

[nukeandbeans]

Entering play mode in the editor breaks lightmap UVs, and re-baking them or re-building the model does nothing to fix it. This is latest build (including #163) on unity 2019.2.12f1

Steps to reproduce:

• Create new model and add a few brushes
• Generate lightmap UVs
• Bake lighting
• Exit play mode

The baked lighting should now be broken.

[nukeandbeans]

Adding to this, restarting the editor clears the lightmaps and allows them to be baked correctly again before pressing play. Immediately after play is pressed, the problem begins again until the next editor restart.