Bug fix: Create OpenGL context only after initializing overlay

- Create OpenGL context only after initializing overlay, which fixes a
  bug that --paint-on-overlay does not work with OpenGL VSync. Thanks to
  tsmithe for reporting. (#7)

compton.c

@@ -5863,6 +5863,14 @@ session_init(session_t *ps_old, int argc, char **argv) {
       ps->o.dbe = false;
   }
 
+  // Overlay must be initialized before double buffer, and before creation
+  // of OpenGL context.
+  if (ps->o.paint_on_overlay)
+    init_overlay(ps);
+
+  if (ps->o.dbe && !init_dbe(ps))
+    exit(1);
+
   // Initialize software optimization
   if (ps->o.sw_opti)
     ps->o.sw_opti = swopti_init(ps);
@@ -5871,13 +5879,6 @@ session_init(session_t *ps_old, int argc, char **argv) {
   if (!vsync_init(ps))
     exit(1);
 
-  // Overlay must be initialized before double buffer
-  if (ps->o.paint_on_overlay)
-    init_overlay(ps);
-
-  if (ps->o.dbe && !init_dbe(ps))
-    exit(1);
-
   // Create registration window
   if (!ps->reg_win && !register_cm(ps))
     exit(1);