This way memory overhead caused by the BVH building is not so visible and peak
memory usage will be reduced.
Implementing this idea is not so straightforward actually, because we need to
synchronize images used for true displacement before meshes. Detecting whether
image is used for true displacement is not so striaghtforward, so for now all
all displacement types will synchronize images used for them.
Such change brings memory usage from 4.1G to 4.0G with the 01_01_01_D scene
from gooseberry. It's not so much in this particular case but it more tricky
scenes benefit could easily be much better.