Mozilla: use --no-xlib

It will be hard to convince all Mozilla plugins-capable browsers to
call XInitThreads() early enough, if it ever happens. In the mean time,
make sure we use only pure XCB and avoid Xlib in VLC plugins.

Note: I did not check that the web plugins itself was not using Xlib
outside the main Mozilla browser thread.

This should fix #3662. We loose GLX and PulseAudio inside Mozilla.
(cherry picked from commit 9886f288158ad2a56d8e2d66910ccde2c87e7206)

projects/mozilla/vlcplugin.cpp

@@ -361,6 +361,7 @@ NPError VlcPlugin::init(int argc, char* const argn[], char* const argv[])
     ppsz_argv[ppsz_argc++] = "--no-media-library";
     ppsz_argv[ppsz_argc++] = "--intf=dummy";
     ppsz_argv[ppsz_argc++] = "--no-video-title-show";
+    ppsz_argv[ppsz_argc++] = "--no-xlib";
 
     const char *progid = NULL;