`Texture.Save` / image encode routes through GDI+ (`System.Drawing`) — throws `PlatformNotSupportedException` on Linux/.NET 7+

Release Type: Official Release
Version: 4.3.0.2507
Platform(s): Linux (and any non-Windows)

Describe the bug
StandardImageHelper.Windows.cs implements PNG/JPG/BMP save via System.Drawing, which is
Windows-only since .NET 6 (hard PlatformNotSupportedException since .NET 7). Any
Texture.Save(stream, ImageFileType.Png) — screenshots, tooling, tests — is dead on Linux.

To Reproduce

1. Linux game, texture.Save(stream, ImageFileType.Png) → PlatformNotSupportedException.

Expected behavior
Cross-platform image save for at least PNG.

Additional context
Fix shape: a managed encoder path for non-Windows — stb-style single-file encoders
(e.g. StbImageWriteSharp, MIT) avoid heavy dependencies; ImageSharp is not an option post-4.0
license change. We ship StbImageWriteSharp app-side for all captures; happy to PR the same
into StandardImageHelper behind an OS check.

Start at StandardImageHelper.Windows.cs and trace the Texture.Save(stream, ImageFileType.Png) path to confirm where System.Drawing is used. Compare the app-side StbImageWriteSharp capture path mentioned in the issue, then verify that PNG saving works on Linux without PlatformNotSupportedException; broader JPG/BMP coverage is not specified.