Misc: Add DEBUG_GLX_MARK & Misc

- GLX backend: Add DEBUG_GLX_MARK, to add GL marks around functions with
  glStringMarkerGREMEDY(), and mark frame termination with
  glFrameTerminatorGREMEDY().

- Print output of `compton -h` to stdout. (#110)

- GLX backend: Strip out elements with factor 0 in GLSL blur code.
  Thanks to jrfonseca for guides. (#107)

src/compton.h

@@ -813,7 +813,7 @@ ev_window(session_t *ps, XEvent *ev);
 #endif
 
 static void __attribute__ ((noreturn))
-usage(void);
+usage(int ret);
 
 static bool
 register_cm(session_t *ps);