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Pointcloud selection support

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.

{F11652666}

Addresses T92415

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D13059