Issue: the texture atlas is being regenerated by seeing the textures on screen. But when both textures are generated, only one callback is returned.
Steps to reproduce:
1. Open attached "ReproProject" project
2. Open "demo_start_scene (more visible)"
3. Go into play mode
-Observe both textures are generated, only one callback is returned.

Note: Both callbacks get called if the prefab that references the font is selected in the editor.

Reproduce the expected result:
1. Re-import both fonts in "fonts" folder
2. Select one of the prefabs in "Prefab" folder
3. Go into play mode
-The callback has been called twice(not once).

Expected result: Font.textureRebuilt should always be called when textures are generated
Actual result: Font.textureRebuilt doesn't get called the second time.

Reproduced on: 5.6.2p4, 2017.1.0p1, 2017.2.0b6, 2017.3.0a3