Picks the entity under the pointer, through the screen camera whose viewport holds it, the way
the editor picks: an id buffer the size of the canvas is rendered for that camera, which draws
into its own viewport of it, and read back under the pointer.
Picks the entity under the pointer, through the screen camera whose viewport holds it, the way the editor picks: an id buffer the size of the canvas is rendered for that camera, which draws into its own viewport of it, and read back under the pointer.