Depth pick translucent primitives

[pjcozzi]

To avoid an extra render pass, scene. pickPosition uses the previous frame's depth buffer, which does not include translucent objects. Sometimes it is useful to pick translucent objects for camera control, e.g., when inside a building, the users wants to drag a window, not the terrain outside the window.

Perhaps we can add an option to render the translucent objects for depth picking.

[pjcozzi]

CC @pmconne

[pjcozzi]

Fixed in #4979.