[2.1.17] plant overlay interactions for tree growth

[mystify]

if the growing plant has an overlay defined in its variations, this simply grows in scale over time as the tree grows, so if it has any relationship to the leaves, it will be misaligned until the tree fully grows. There should be tree growth settings for the overlay as well, or if that doesn't play nicely with the rendering setup, being able to at least set a threshold when the overlay appears would be helpful.

I'm using the overlay to display fruit growing on the tree. If I include them as part of the leaves, their color gets washed out due to the leaf tinting, so having it as an overlay keeps the vibracy.