Fix T76729: Particle Hair Get Occluded by Overlays

When using an external render engine Blender will still draw an OpenGL
depth buffer for the overlay engine to work correctly. Particle systems
were ignored, what lead to occluded hair and other artifacts.

Reviewed By: Clément Foucault

Differential Revision: https://developer.blender.org/D7730

source/blender/draw/engines/external/external_engine.c

@@ -25,10 +25,12 @@
 
 #include "DRW_render.h"
 
+#include "DNA_modifier_types.h"
 #include "DNA_screen_types.h"
 #include "DNA_view3d_types.h"
 
 #include "BKE_object.h"
+#include "BKE_particle.h"
 
 #include "ED_screen.h"
 
@@ -166,6 +168,24 @@ static void external_cache_populate(void *vedata, Object *ob)
     return;
   }
 
+  if (ob->type == OB_MESH && ob->modifiers.first != NULL) {
+    LISTBASE_FOREACH (ModifierData *, md, &ob->modifiers) {
+      if (md->type != eModifierType_ParticleSystem) {
+        continue;
+      }
+      ParticleSystem *psys = ((ParticleSystemModifierData *)md)->psys;
+      if (!DRW_object_is_visible_psys_in_active_context(ob, psys)) {
+        continue;
+      }
+      ParticleSettings *part = psys->part;
+      const int draw_as = (part->draw_as == PART_DRAW_REND) ? part->ren_as : part->draw_as;
+
+      if (draw_as == PART_DRAW_PATH) {
+        struct GPUBatch *hairs = DRW_cache_particles_get_hair(ob, psys, NULL);
+        DRW_shgroup_call(stl->g_data->depth_shgrp, hairs, NULL);
+      }
+    }
+  }
   struct GPUBatch *geom = DRW_cache_object_surface_get(ob);
   if (geom) {
     /* Depth Prepass */