Now `ED_view3d_backbuf_depth_validate`, `ED_view3d_draw_depth` and `ED_view3d_draw_depth_gpencil` are unified in `ED_view3d_depth_override`. This new function replaces `ED_view3d_autodist_init`. Also, since `ED_view3d_depth_update` depends on the render context, and changing the context is a slow operation, that function also was removed, and the depth buffer cached is now updated inside the new unified drawing function when the "bool update_cache" parameter is true. Finally `V3D_INVALID_BACKBUF` flag has been renamed and moved to `runtime.flag`. Differential revision: https://developer.blender.org/D10678