avidemux: 2.7.0 -> 2.7.1

The current version of `avidemux` (v2.7.0) breaks on master (see
https://hydra.nixos.org/build/75993794), the 18.03 build remains fine
(https://hydra.nixos.org/build/75935027).

The following changes have been applied:

* `glibc` compat patch is obsolete as it landed in upstream.

* Fixed QT_PLUGIN_PATH for QT binaries to avoid errors because of
  missing plugins like this:

  ```
  qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in ""
  This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

  [1]    20081 abort      /nix/store/rjwxc6ih15zwvvy71ss8bvnh56ibfbmj-avidemux-2.7.1/bin/avidemux3_qt5
  ```