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.
Misc #218: Warn about using --glx-no-rebind-pixmap on intel driver

Warn about using --glx-no-rebind-pixmap on xf86-video-intel. Thanks to
aktau for the info. (#218)
Misc: Fix compilation with NO_VSYNC_OPENGL_GLSL / NO_C2

Fix compilation with NO_VSYNC_OPENGL_GLSL / NO_C2.