[GIMP](http://www.gimp.org/) is a strong alternative.

Gimp is available on OS X, Windows, and Linux. Gimp is free and licensed under the [GNU General Public License](http://www.gimp.org/about/COPYING).

![GIMP][1]


  [1]: https://i.sstatic.net/ycAkg.png

As for your feature list:

 - layers

    Yes, gimp has this.

 - all layer effects

    Yes

 - patterns

    Yes

 - gradients

    Yes

 - smart objects

    Gimp does not have smart objects to my knowledge.

 - history

    Yes

 - PNG export

    Yes

 - rescaling

    Yes

 - text (and font properties)- paths

    Yes

 - shapes

    Yes

 - selections / color selecting

    Yes  

 - rotation  

    Yes