Bug 1827137 - Allow overriding CSD support with GTK_CSD=0, so that gtk3-nocsd keeps working. r=stransky

I don't think there's a particularly nicer/easier way of doing this,
wdyt?

Differential Revision: https://phabricator.services.mozilla.com/D175061

widget/gtk/nsWindow.cpp

@@ -9323,9 +9323,8 @@ nsWindow::GtkWindowDecoration nsWindow::GetSystemGtkWindowDecoration() {
     // GTK_CSD forces CSD mode - use also CSD because window manager
     // decorations does not work with CSD.
     // We check GTK_CSD as well as gtk_window_should_use_csd() does.
-    const char* csdOverride = getenv("GTK_CSD");
-    if (csdOverride && *csdOverride == '1') {
-      return GTK_DECORATION_CLIENT;
+    if (const char* csdOverride = getenv("GTK_CSD")) {
+      return *csdOverride == '0' ? GTK_DECORATION_NONE : GTK_DECORATION_CLIENT;
     }
 
     // TODO: Consider switching this to GetDesktopEnvironmentIdentifier().