Add support for reading indices of palletized pngs'

[benbaker76]

There are many reasons why someone might need to read the raw indices of a png. I've written tools and applications in various languages (C#, Python and C) for various platforms and API's (currently Spectrum Next and DAVE2D) where I need access to the raw index data.

ImageSharp supports reading indexed pngs' but converts the indices to a color format which is destructive. Once converted you no longer know which color in the palette a pixel is. Also a platform such as DAVE2D supports palletized formats directly and a conversion back to indices would be required. And finally this doesn't work because there are often repeating colors in a palette such as for when a png contains 16 4-bit color palettes in an 8-bit palette.

Please add PixelFormats.Index1, PixelFormats.Index2, PixelFormats.Index4 and PixelFormats.Index8 formats and copy the indices directly from the png data.

[benbaker76]

I recommend people who need to use pngs' in a non-destructive way to use the pngcs library.