(PointCloudLayer) add points filtre based on classification

[ftoromanoff]

Any classification scheme used for PointCloudLayer havea property 'visible' for each class.
This PR aims at using this value to filter point whichever the visualization mode choosen.

In other word, if a class has its property 'visible' set to false, we want to discard all points having the value in the rendering, (even if we choose to use the 'intensity' rendering)

In this PR, I used migrate all PointCloud examples to lilGUI instead of datGUI

[HoloTheDrunk]

I see we're using a conditional discard in the fragment shader; it might be worth benchmarking the performance of this against using a transparent color or performing the filtering earlier on CPU (which, given the right optimization work, might turn out to be the best solution).
See this (old) SO post for an explanation: https://stackoverflow.com/questions/8509051/is-discard-bad-for-program-performance-in-opengl
This is always going to depend on each user's hardware anyways, but checking wouldn't hurt if we can gain a bit of performance.

[Desplandis]

Need another review.