Fix sprite margin transparency for Photoshop

For some weird reason Photoshop converts the `ChunkyPNG::Color::TRANSPARENT` color `#00000000` into `#000000`, which shows as black. Because of this it is impossible to edit the generated sprites (to for instance create a white version of a black icon set).

I fixed this by replacing the `ChunkyPNG::Color::TRANSPARENT` color with the transparent color `ChunkyPNG::Color.rgba(255, 255, 255, 0)` (`#ffffff00`), which Photoshop treats correctly.

I also updated the integration tests to reflect this change.