tools: gst-launch: set GST_GL_XINITTHREADS

This ensure that XInitThreads is called and so gl contexts are properly
initialized.

https://bugzilla.gnome.org/show_bug.cgi?id=776401

tools/gst-launch.c

@@ -1005,6 +1005,8 @@ main (int argc, char *argv[])
 #endif
 
   g_set_prgname ("gst-launch-" GST_API_VERSION);
+  /* Ensure XInitThreads() is called if/when needed */
+  g_setenv ("GST_GL_XINITTHREADS", "1", TRUE);
 
 #ifndef GST_DISABLE_OPTION_PARSING
   ctx = g_option_context_new ("PIPELINE-DESCRIPTION");