--no-xlib: prevent use of XInitThreads() and hence Xlib

Certain LibVLC applications fail to call XInitThreads() before they
call XOpenDisplay(). Then VLC calls XInitThreads(). Then the
applications call XCloseDisplay(), which raises a segmentation fault.
In this case, Xlib tries acquire lock that was never created as the
Display ppinter was created before threaded Xlib mode was enabled.

These applications can now pass --no-xlib to libvlc_new(). This will
prevent any VLC Xlib-based plugin from being used. Currently, this
affects interfaces (not really an issue), PulseAudio (until Colin's
patch is merged upstream) and GLX (Xlib-based by design).

This will be necessary to address #3662.
(cherry picked from commit 95c95cc9)

modules/audio_output/pulse.c

@@ -124,7 +124,7 @@ static int Open ( vlc_object_t *p_this )
 #ifdef X_DISPLAY_MISSING
 # error Xlib required due to PulseAudio bug 799!
 #else
-    if( !XInitThreads() )
+    if( !var_InheritBool( p_this, "xlib" ) || !XInitThreads() )
         return VLC_EGENERIC;
 #endif
     /* Allocate structures */

modules/gui/hildon/maemo.c

@@ -80,7 +80,7 @@ static int Open( vlc_object_t *p_this )
     intf_sys_t *p_sys;
     vlc_value_t val;
 
-    if( !XInitThreads() )
+    if( !var_InheritBool( p_this, "xlib" ) || !XInitThreads() )
         return VLC_EGENERIC;
 
     /* Allocate instance and initialize some members */

modules/gui/qt4/qt4.cpp

@@ -283,7 +283,7 @@ static int Open( vlc_object_t *p_this, bool isDialogProvider )
     intf_thread_t *p_intf = (intf_thread_t *)p_this;
 
 #ifdef Q_WS_X11
-    if( !XInitThreads() )
+    if( !var_InheritBool( p_this, "xlib" ) || !XInitThreads() )
         return VLC_EGENERIC;
 
     char *display = var_CreateGetNonEmptyString( p_intf, "x11-display" );

modules/gui/skins2/x11/x11_factory.cpp

@@ -57,8 +57,11 @@ X11Factory::~X11Factory()
 bool X11Factory::init()
 {
     // make sure xlib is safe-thread
-    if( !XInitThreads() )
+    if( !var_InheritBool( getIntf(), "xlib" ) || !XInitThreads() )
+    {
         msg_Err( getIntf(), "initializing xlib for multi-threading failed" );
+        return false;
+    }
 
     // Create the X11 display
     m_pDisplay = new X11Display( getIntf() );

modules/video_output/xcb/glx.c

@@ -207,7 +207,7 @@ static int CreateWindow (vout_display_t *vd, xcb_connection_t *conn,
  */
 static int Open (vlc_object_t *obj)
 {
-    if (!XInitThreads ())
+    if (!var_InheritBool (obj, "xlib") || !XInitThreads ())
         return VLC_EGENERIC;
 
     vout_display_t *vd = (vout_display_t *)obj;

src/libvlc-module.c

@@ -1641,6 +1641,8 @@ vlc_module_begin ()
                 DISPLAY_TEXT, DISPLAY_LONGTEXT, true )
         add_deprecated_alias( "xvideo-display" ) /* deprecated since 1.1.0 */
         add_deprecated_alias( "glx-display" )
+    add_bool( "xlib", true, NULL, "", "", true )
+        change_private ()
     add_bool( "drop-late-frames", 1, NULL, DROP_LATE_FRAMES_TEXT,
               DROP_LATE_FRAMES_LONGTEXT, true )
     /* Used in vout_synchro */