WebP: Lossy and transparency

[PhyxionNL]

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

I'm unable to get lossy and transparency to work together. In lossless mode, it works OK, but in lossy the alpha channel is always rendered as black. I've experimented with the TransparentColorMode = WebpTransparentColorMode.Preserve option but that didn't help. According to Google ( https://developers.google.com/speed/webp/gallery2 ) it should be possible to combine alpha with lossy mode.

Steps to Reproduce

  using var img = SixLabors.ImageSharp.Image.Load("in.png"); // Something with transparency.

  using var fs = new FileStream("out.webp", FileMode.OpenOrCreate);

  img.Save(fs, new WebpEncoder { FileFormat = WebpFileFormatType.Lossy});

System Configuration

• ImageSharp version: 2.0 alpha 99
• Other ImageSharp packages and versions: -
• Environment (Operating system, version and so on): Windows 10.
• .NET Framework version: .NET 5.0 / 6.0.

[PhyxionNL]

E: It's in #1802 already, somehow it didn't show up in search. Closing...