Refactor: Do not keep a copy of depth buffer in RegionView3D

The depth cache (located in `RegionView3D::depths`) is used for quick
and simple occlusion testing in:
- particle selection,
- "Draw Curve" operator and
- "Interactive Light Track to Cursor" operator,

However, keeping a texture buffer in cache is not a recommended practice.

For displays with high resolution like 8k this represents something
around 132MB.

Also, currently, each call to `ED_view3d_depth_override` invalidates
the depth cache. So that depth is never reused in multiple calls from
an operator (this was not the case in blender 2.79).

This commit allows to create a depth cache and release it in the same
operator. Thus, the buffer is kept in cache for a short time, freeing
up space.

No functional changes.

source/blender/blenkernel/intern/screen.c

@@ -1467,7 +1467,6 @@ static void direct_link_region(BlendDataReader *reader, ARegion *region, int spa
         BLO_read_data_address(reader, &rv3d->localvd);
         BLO_read_data_address(reader, &rv3d->clipbb);
 
-        rv3d->depths = NULL;
         rv3d->render_engine = NULL;
         rv3d->sms = NULL;
         rv3d->smooth_timer = NULL;