[godot] Add CPU backface culling

[badlogic]

See
https://esotericsoftware.com/forum/d/27102-godot%E4%B8%AD%E5%BC%80%E5%90%AFbackface-culling/5
https://en.esotericsoftware.com/forum/d/23925-godot-spine-backface-culling-feature/10

Godot currently does not support GPU side backface culling in 2D. We need to do that on the CPU. Since this will eat some CPU cycles, it should be off by default. Users should be able to enable it via a property on SpineSprite