libpng warns about invalid chunks

[jansol]

I recently got these warnings after recompressing a set of sprites with oxipng 0.12.0:

libpng warning: sBIT: invalid
libpng warning: bKGD: invalid

The files were compressed with oxipng -o6 -Z sample$N-good.png --out sample$N-bad.png

Some of the offending files:
Bad:
Good:
Bad:
Good:
Bad:
Good:
Bad:
Good:
Bad:
Good:
Bad:
Good:
(I hope GitHub didn't mangle those)

[shssoichiro]

Thanks for the bug report! I'll take a look at these this evening after work.