Destroy cubemaps with the scene.

src/scene.nim

@@ -351,6 +351,11 @@ proc destroy*(self: Scene) =
         self.shadow_maps.destroy()
     for ubo in self.lighting_UBOs:
         ubo.destroy()
+    if self.background_cubemap != nil:
+        self.background_cubemap.destroy()
+    for cube in self.cubemaps:
+        cube.destroy()
+    self.cubemaps = @[]
     self.engine.scenes.del(self.name)
     # bound textures can linger
     unbindAllTextures()