Fix: GPv3 depth buffer

The depth buffer was rendered to, but not correctly merged
with the scene depth buffer. This lead to, e.g. the object appearing
behind the grid.

This fixes the issue by rendering a "merge" pass.

@@ -91,6 +92,7 @@ class ObjectModule {
   void begin_sync(Depsgraph *depsgraph, const View &main_view)
   {
     camera_forward_ = float3(main_view.viewinv()[2]);
+    camera_pos_ = float3(main_view.viewinv()[3]);

@@ -252,0 +286,4 @@
+    float4x4 bbox_mat = object_to_world *
+                        from_loc_rot_scale<float4x4>(center, Quaternion::identity(), size);
+    float3 plane_normal;
+    if (DRW_view_is_persp_get(nullptr)) {