Fix #2779 buffer overrun

[KirillAldashkin]

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Description

Fix #2779 by accounting pixel size. Also fixed some width*height multuplication overflows:

using SixLabors.ImageSharp;
using SixLabors.ImageSharp.PixelFormats;

Rgba64[] data = new Rgba64[65536];
using Image<Rgba64> img = Image.WrapMemory<Rgba64>(data, 65537, 65536);
// doesn't throw here but will blow up later

// ...here for example
img.SaveAsPng("output.png");

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[JimBobSquarePants]

This all looks great. Thanks so much for your help!