Bugfix/#779 smartsheet not published at first

Fixes https://github.com/spritebuilder/SpriteBuilder/issues/779

This request fixes another issue that turned up while testing:
* Create a smart sprite sheet
* Change format to PVRRGBA8888
* Publish
* Change format to PNG
* Publish

PNG is not exported. This was caused by some projectSettings internals that did not mark certain values as dirty.