Update AssetManager Bitmap font register and cleanup logic.

[soimy]

Update proccessXml logic mentioned at #997 :

• remove texture after all bitmap font registered.
• use font-face name instead of texture name as bitmap font name.

[PrimaryFeather]

Thanks a lot for creating the pull request! You're right, that's what needs to be done. I'll review and add that next week!

[PrimaryFeather]

I implemented this change when closing #997.