Adding interlaced PNG support to image/png

[Luke Granger-Brown]

Hello all,

I'm using image/png in a public facing way and people keep uploading interlaced PNGs (bah). I have essentially my own fork of image/png at github.com/lukegb/go-image-png/png which supports reading (but not writing) interlaced PNGs but wanted to check if my changes were sound and sane to submit before I go through the hurdles required to submit a proper patchset for code review - I originally wrote these changes with just my personal use in mind but quickly realised they might be of use to the community at large!

Issue: https://code.google.com/p/go/issues/detail?id=6293

Luke

[minux]

The issue is accepted, so your changes are definitely welcome!

However, as the tree is in feature freeze right now, please send the CL

after Go 1.3 is released (estimated to be on June 1st).