Fix Cloth Brush global Sculpt gravity applied in the wrong falloff

Previously, gravity was only applied in the real brush radius, not in
the whole simulation radius. For most deformation modes, applying
gravity to the entire simulation instead of just to the brush radius and
scaled by the radius (like a regular sculpt brush) makes more sense.
After this fix and with the cloth collisions patch applied, it is possible
to do things like this with the cloth grab brush.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8406

source/blender/editors/sculpt_paint/sculpt_cloth.c

@@ -254,8 +254,7 @@ static void do_cloth_brush_apply_forces_task_cb_ex(void *__restrict userdata,
   /* Gravity */
   float gravity[3] = {0.0f};
   if (ss->cache->supports_gravity) {
-    madd_v3_v3fl(
-        gravity, ss->cache->gravity_direction, -ss->cache->radius * data->sd->gravity_factor);
+    madd_v3_v3fl(gravity, ss->cache->gravity_direction, -data->sd->gravity_factor);
   }
 
   /* Original data for deform brushes. */
@@ -279,6 +278,11 @@ static void do_cloth_brush_apply_forces_task_cb_ex(void *__restrict userdata,
       copy_v3_v3(current_vertex_location, vd.co);
     }
 
+    /* Apply gravity in the entire simulation area. */
+    float vertex_gravity[3];
+    mul_v3_v3fl(vertex_gravity, gravity, sim_factor);
+    cloth_brush_apply_force_to_vertex(ss, ss->cache->cloth_sim, vertex_gravity, vd.index);
+
     /* When using the plane falloff mode the falloff is not constrained by the brush radius. */
     if (sculpt_brush_test_sq_fn(&test, current_vertex_location) || use_falloff_plane) {
 
@@ -356,8 +360,6 @@ static void do_cloth_brush_apply_forces_task_cb_ex(void *__restrict userdata,
           break;
       }
 
-      madd_v3_v3fl(force, gravity, fade);
-
       cloth_brush_apply_force_to_vertex(ss, ss->cache->cloth_sim, force, vd.index);
     }
   }