PngDecoder should not throw when encountering invalid Zlib data for non critical chunks.

[JimBobSquarePants]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

Description

The following image will cause our decoder to throw due to an invalid Zlib data contained within the zTxt chunk. While correct, this chunk is non-critical and will be ignored during decoding by libpng and System.Drawing.

Steps to Reproduce

System Configuration

• ImageSharp version:
• Other ImageSharp packages and versions: 1.0.0-beta0007
• Environment (Operating system, version and so on): All
• .NET Framework version: All
• Additional information: