Dev:CgBI file format

The CgBI file format, thus named for its extra header, is Apple's proprietary extension to the PNG image format.

Differences from PNG
These modifications cause the generated images to be invalid as per the current version of the PNG standard.
 * extra critical chunk (CgBI)
 * byteswapped (RGBA -> BGRA) pixel data, presumably for high-speed direct blitting to the framebuffer
 * zlib header, footer, and CRC removed from the IDAT chunk
 * premultiplied alpha (color' = color * alpha / 255)

To account for or apply these modifications, libpng needs to be patched to accept the "invalid" zlib compression window (a 15-bit window is the default, supplying -15 to zlib will cause it to omit headers/footers/CRC info).

CgBI Chunk
CgBI is a four-byte critical chunk added to the beginning of all "optimized" PNG files.

Known Contents
At least the lower parts of the content correspond to the documented CGBitmapInfo bitmask.
 * 0x50 0x00 0x20 0x06
 * 0x50 0x00 0x20 0x02 (seen if input image was already RGBA)

Encoding

 * Apple's modified pngcrush (xcrun -sdk iphoneos pngcrush -iphone)
 * pincrush

Decoding

 * Apple's modified pngcrush (xcrun -sdk iphoneos pngcrush -revert-iphone-optimizations)
 * ipin.py ("iPhone PNG Image Normalizer")