This patch adds support for selecting pointclouds.

Since pointclouds were not properly drawn to the selection buffer (as diagonsed by output from glReadPixels and Renderdoc), they were not able to be selectable by depth picking or occlusion queries. In basic_engine, objects were rendered with a shader which draws to a depth buffer but only assumes a single position vertex attribute. Pointclouds, though, require at least another vertex attribute pos_inst which provides the instance offsets. Thus, this patch adds another shader variant for pointclouds which supports these two attributes and renders the points appropriately.

Addresses T92415