Describe the bug
One of recent packages updates broke XEmbed tray icons on MATE, they lost transparency and have other graphical artifacts. SNI tray icons displayed by the same applet seem to work fine at the same time.

To Reproduce
Steps to reproduce the behavior:

1. Install MATE on NixOS unstable
2. Probably nothing else?

Expected behavior
No graphical artifacts

Screenshots

Notify maintainers
@romildo

Metadata

• system: "x86_64-linux"
• host os: Linux 5.11.16-zen1, NixOS, 21.05pre-git (Okapi)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.10
• nixpkgs: /run/nixpkgs

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: mate.mate-panel
# a list of nixos modules affected by the problem
module: services.xserver.desktopManager.mate