remove index8 check for adding pixels
remove index8 check for adding pixels

After looking at this there really isn't any reason that this check
needs to be here. It's up to the caller to ensure that it's safe though.

mono/SkiaSharp,mono/SkiaSharp,mono/SkiaSharp,mono/SkiaSharp,mono/SkiaSharp,mono/SkiaSharp,mono/SkiaSharp