Improvement: Separate GLX parts from session_t & Attempt to fix #217

- Separate GLX parts from session_t into glx_session_t.

- Add --rererdir-on-root-change and --glx-reinit-on-root-change, as
  possible solutions for #217. Thanks to jlindgren90 for reporting.

src/opengl.h

@@ -41,7 +41,7 @@ glx_dump_err_str(GLenum err) {
  */
 static inline void
 glx_check_err_(session_t *ps, const char *func, int line) {
-  if (!ps->glx_context) return;
+  if (!ps->psglx->context) return;
 
   GLenum err = GL_NO_ERROR;