ABI bump required?

[roehling]

Going from 1.5.0 to 1.5.1, the SceneNode class gained a new member light_index_, which changes the object size, and the Scene class gained a new member lights_, also changing the object size and the internal offset of material_library_.

[tomfinegan]

This (and #815) is why 1.5.2 is on the way. Should be available in a few hours.