Fix Decoding 8-Bit interlaced grayscale png's with alpha channel#830

JimBobSquarePants merged 2 commits intoSixLabors:masterSixLabors/ImageSharp:masterfrom

brianpopow:feature/fixDecodePng8bitWithAlphabrianpopow/ImageSharp:feature/fixDecodePng8bitWithAlphaCopy head branch name to clipboard