Added iDOT, now it is registered

[svgeesus]

Added minimal support (just so that an error is not reported) given that iDOT is now in the extensions document.

https://w3c.github.io/png/extensions/Overview.html#R.iDOT

$ pngcheck -c -v apple-iDOT.png
File: apple-iDOT.png (69252 bytes)
  chunk IHDR at offset 0x0000c, length 13
    1192 x 280 image, 32-bit RGB+alpha, non-interlaced
  chunk sRGB at offset 0x00025, length 1
    rendering intent = perceptual
  chunk eXIf at offset 0x00032, length 150: EXIF metadata, big-endian (MM) format
  chunk pHYs at offset 0x000d4, length 9: 5669x5669 pixels/meter (144 dpi)
  chunk iTXt at offset 0x000e9, length 471, keyword: XML:com.adobe.xmp
    uncompressed, no language tag
    no translated keyword, 450 bytes of UTF-8 text
  chunk iDOT at offset 0x002cc, length 28: Parallel segments: 2
  chunk IDAT at offset 0x002f4, length 16384
    zlib: deflated, 32K window, superfast compression
  chunk IDAT at offset 0x04300, length 11208
  chunk IDAT at offset 0x06ed4, length 16384
  chunk IDAT at offset 0x0aee0, length 16384
  chunk IDAT at offset 0x0eeec, length 8068
  chunk IEND at offset 0x10e7c, length 0
No errors detected in apple-iDOT.png (12 chunks, 94.8% compression).

• Add the Apple iDOT chunk to the PNG Extensions specification w3c/png#530

[svgeesus]

@ctruta

[ctruta]

LGTM 👍