Blender Git Loki

Mesh Selection: Move Selection ID Context Utilities to ED_view3d.

This patch does not bring functional changes, but it is a good change if we want to use these utilities in areas other than those using BMesh (eg painting editors).

This is also a step to replace `ED_view3d_select_id_validate`. That function erroneously checks `V3D_INVALID_BACKBUF` which causes it to update unnecessarily.

Reviewers: campbellbarton

Differential Revision: https://developer.blender.org/D5072