Don't load webkit extensions from CWD-relative paths

wxgtk_initialize_web_extensions() tries to find extensions in the
location where "make install" puts them. If that fails, either because
wx wasn't installed yet, or it was deployed (e.g. in static build)
without them, it looks into additional places - paths relative to CWD.
This is to faciliate running wx samples and tests, but by being enabled
in release builds too, it allowed loading unexpected executable code
from untrusted locations. For example, on typical desktops, one could
put malicious .so files into $HOME/lib to have them loaded by
wxWebView-using applications.

Address this by making the helper paths relative to the executable's
location, not working directory.

src/gtk/webview_webkit2.cpp

@@ -14,6 +14,7 @@
 #include "wx/dir.h"
 #include "wx/dynlib.h"
 #include "wx/filename.h"
+#include "wx/stdpaths.h"
 #include "wx/stockitem.h"
 #include "wx/gtk/webview_webkit.h"
 #include "wx/gtk/control.h"
@@ -399,6 +400,15 @@ static bool CheckDirectoryForWebExt(const wxString& dirname)
     return false;
 }
 
+static bool TrySetWebExtensionsDirectory(WebKitWebContext *context, const wxString& dir)
+{
+    if (dir.empty() || !CheckDirectoryForWebExt(dir))
+        return false;
+
+    webkit_web_context_set_web_extensions_directory(context, dir.utf8_str());
+    return true;
+}
+
 static wxString GetStandardWebExtensionsDir()
 {
     wxString dir = wxDynamicLibrary::GetPluginsDirectory();
@@ -414,23 +424,27 @@ wxgtk_initialize_web_extensions(WebKitWebContext *context,
     const char *address = g_dbus_server_get_client_address(dbusServer);
     GVariant *user_data = g_variant_new("(s)", address);
 
-    // The first value is the location in which the extension is supposed to be
-    // normally installed, while the other three are used as fallbacks to allow
-    // running the tests and sample using wxWebView before installing it.
-    wxString const directories[] =
+    // Try to setup extension loading from the location it is supposed to be
+    // normally installed in.
+    if ( !TrySetWebExtensionsDirectory(context, GetStandardWebExtensionsDir()) )
     {
-        GetStandardWebExtensionsDir(),
-        "..",
-        "../..",
-        "lib",
-    };
-
-    for ( size_t n = 0; n < WXSIZEOF(directories); ++n )
-    {
-        if ( !directories[n].empty() && CheckDirectoryForWebExt(directories[n]) )
+        // These relative locations are used as fallbacks to allow running
+        // the tests and sample using wxWebView before installing it.
+        wxString exepath = wxFileName(wxStandardPaths::Get().GetExecutablePath()).GetPath();
+        if ( !exepath.empty() )
         {
-            webkit_web_context_set_web_extensions_directory(context, directories[n].utf8_str());
-            break;
+            wxString const directories[] =
+            {
+                exepath + "/..",
+                exepath + "/../..",
+                exepath + "/lib",
+            };
+
+            for ( size_t n = 0; n < WXSIZEOF(directories); ++n )
+            {
+                if ( !TrySetWebExtensionsDirectory(context, directories[n]) )
+                    break;
+            }
         }
     }