libkscreen: hardcode backends path

libkscreen provides all available kscreen backends, so there is no reason to
search the environment for plugins.

pkgs/desktops/plasma-5/default.nix

@@ -62,7 +62,7 @@ let
       kwayland-integration = callPackage ./kwayland-integration.nix {};
       kwin = callPackage ./kwin {};
       kwrited = callPackage ./kwrited.nix {};
-      libkscreen = callPackage ./libkscreen.nix {};
+      libkscreen = callPackage ./libkscreen {};
       libksysguard = callPackage ./libksysguard {};
       milou = callPackage ./milou.nix {};
       oxygen = callPackage ./oxygen.nix {};

pkgs/desktops/plasma-5/libkscreen.nix

@@ -1,15 +0,0 @@
-{ plasmaPackage
-, extra-cmake-modules
-, kwayland, libXrandr
-, qtx11extras
-}:
-
-plasmaPackage {
-  name = "libkscreen";
-  nativeBuildInputs = [
-    extra-cmake-modules
-  ];
-  propagatedBuildInputs = [
-    kwayland libXrandr qtx11extras
-  ];
-}

pkgs/desktops/plasma-5/libkscreen/default.nix

@@ -0,0 +1,19 @@
+{ plasmaPackage, lib, copyPathsToStore
+, extra-cmake-modules
+, kwayland, libXrandr
+, qtx11extras
+}:
+
+plasmaPackage {
+  name = "libkscreen";
+  nativeBuildInputs = [
+    extra-cmake-modules
+  ];
+  propagatedBuildInputs = [
+    kwayland libXrandr qtx11extras
+  ];
+  patches = copyPathsToStore (lib.readPathsFromFile ./. ./series);
+  preConfigure = ''
+    NIX_CFLAGS_COMPILE+=" -DNIXPKGS_LIBKSCREEN_BACKENDS=\"''${!outputLib}/lib/qt5/plugins/kf5/kscreen\""
+  '';
+}

pkgs/desktops/plasma-5/libkscreen/libkscreen-backends-path.patch

@@ -0,0 +1,27 @@
+Index: libkscreen-5.9.4/src/backendmanager.cpp
+===================================================================
+--- libkscreen-5.9.4.orig/src/backendmanager.cpp
++++ libkscreen-5.9.4/src/backendmanager.cpp
+@@ -178,17 +178,11 @@ QFileInfo BackendManager::preferredBacke
+ QFileInfoList BackendManager::listBackends()
+ {
+     // Compile a list of installed backends first
+-    const QString backendFilter = QStringLiteral("KSC_*");
+-    const QStringList paths = QCoreApplication::libraryPaths();
+-    QFileInfoList finfos;
+-    for (const QString &path : paths) {
+-        const QDir dir(path + QLatin1String("/kf5/kscreen/"),
+-                       backendFilter,
+-                       QDir::SortFlags(QDir::QDir::Name),
+-                       QDir::NoDotAndDotDot | QDir::Files);
+-        finfos.append(dir.entryInfoList());
+-    }
+-    return finfos;
++    const QDir dir(QStringLiteral(NIXPKGS_LIBKSCREEN_BACKENDS),
++                   QStringLiteral("KSC_*"),
++                   QDir::SortFlags(QDir::QDir::Name),
++                   QDir::NoDotAndDotDot | QDir::Files);
++    return dir.entryInfoList();
+ }
+ 
+ KScreen::AbstractBackend *BackendManager::loadBackendPlugin(QPluginLoader *loader, const QString &name,

pkgs/desktops/plasma-5/libkscreen/series

@@ -0,0 +1 @@
+libkscreen-backends-path.patch