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Edit Mesh Selection: Refactor: Redraw idmap buffer at runtime with only objects inside the rect

But in the future the selection code may also be used in object mode (eg for snapping).
So to avoid using too much VRAM resources, it is good to avoid drawing all objects in the viewport.

The solution was to create an array with only objects that are detected within the selection area.
If the selection operator is modal, objects already detected are not removed from the array until view3d is moved or orbited.
To detect the object, its BoundBox is tested.
Since the Select Engine does not have a dedicated depth texture, whenever a new object is "found" the depth of the objects in the array already drawn is redrawn.

Reviewers: campbellbarton, fclem

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D5435