Handle version 6 of slices and normalize between versions
Also create mask when #to_png called
Memorize png instance in #to_png and #to_png_with_mask
Oop! revert back the correct #build_png
Fixes slice parsing issue
Apply clipping mask before layer styles
Remove this incorrect blend optimization
Merge branch 'version_6_slices'
Fixes #39 as best as possible. Might need further inspection.
Cleanup