Add 'Selection' bool on 'Realize Instances' node

The concept is pretty straightforward: add a Selection input in order to selectively realize instances .

This would be especially useful in computing/geometry-heavy contexts, where you only need to modify or affect only a small selective subset of the geometry. For example, deformation behavior on grass blades as something move through -- without calling 'realize instances' on 1.7 million points. (this is most straightforward example I had in mind -- I know that in that specific case there are other solutions)

There are workaround for this, such as creating two distinct 'instances on Points', one you realize, the other you don't. However this comes with limitation if you want to use the instance index as a random seed for position/rotation: indices are not the same in the two groups, leading to a "popin" behavior where rotation and position differ from the original setup. A workaround to that is to use position as a seed, but this is not a reliable/stable approach.

Alternatively, this would be useful to only process geometry deformation & node graphs based on active camera' frustum; or simply general optimization in complex setups.