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DrawManager: Crash Face Selection

Init the scene of the draw context when selecting. When using face dot selection on
when the subsurf modifier is active on the cage, the scene needs to be
valid. It is read from the context in the
`DRW_mesh_batch_cache_create_requested` and used in the `isDisabled`
method of the SubSurfModifier.

Reviewers: fclem, sergey

Differential Revision: https://developer.blender.org/D5214