Update DebugMesh when NavMesh changes
Update DebugMesh when NavMesh changes

Fixes #7371
(cherry picked from commit 2807507325c28a4b4f5128db5ee98f9a282fe5d1)

pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1