As part of Fbx Export, while handling Armature modifiers for Meshes, each
armature's position is backed up, then put into REST position for exporting,
and then restored back to original position. A dependency graph update is
triggered at the end of this.

This commit avoids the whole backing position setting + depsgraph update in
case the armature is already in rest position.

As an example, a model which I am developing for a game used to take
20 minutes for the Fbx Export. After this change, it only takes 20 seconds.

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D13712