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
1425411249