PNG package code bloated

The PNG component is really really bloated, and code is written for the same thing twice sometimes. For example, the formats.png package uses the `PngWriter` class which internally replicates the features that should actually be contained within formats.png.chunk Chunk classes (like PngWriter.ImageHeader which is just a PNG chunk representation, and can be eliminated by modified Chunk and PngChunkIhdr).

The file format classes can be modified to reduce the code size significantly and make it much more clear. I'll clean up the PNG code, but want the interest of the project members.