Fix Imagick get size for animated gif

Instead of using last frame and getting size on it, use
first one which always has right width and height.
Preserve iterator state on getSize
Unit test for size of animated gif
Fix Gmagick get size for animated gif
Update CHANGELOG
Merge branch 'master' into develop

* master:
  Update CHANGELOG
  Fix Gmagick get size for animated gif
  Unit test for size of animated gif
  Preserve iterator state on getSize
  Fix Imagick get size for animated gif