PNG support with fast FPNGE encoder

[YorVeX]

FPNGE is only an encoder. Decoding can be done using ImageSharp and in case that doesn't perform well enough later be replaced by a different decoder.

Unfortunately FPNG (without E) doesn't support decoding FPNGE encoded files, so the next best fast decoder would be wuffs, however, implementing this would take a lot of effort. Maybe this adapter code could be compiled into the library instead.