GdkGLContext: Never use glXSwapIntervalSGI on composited WMs

Delaying the copy of the backbuffer to the front buffer until
the vblank doesn't make sense when the frontbuffer is not the
real screen, as the copy will not flicker anyway. It is the
responsibility of the compositor to sync *its* drawing of the
window to the vblank.

The other reason to delay to vblank is to avoid rendering all the
time and sync each frame to the actual frame rate. However, we
already handle this in Gtk+ via the frame clock, so this should
not be needed.

gdk/x11/gdkglcontext-x11.c

@@ -173,6 +173,7 @@ maybe_wait_for_vblank (GdkDisplay  *display,
 static void
 gdk_x11_gl_context_flush_buffer (GdkGLContext *context)
 {
+  GdkX11GLContext *context_x11 = GDK_X11_GL_CONTEXT (context);
   GdkDisplay *display = gdk_gl_context_get_display (context);
   GdkWindow *window = gdk_gl_context_get_window (context);
   Display *dpy = gdk_x11_display_get_xdisplay (display);
@@ -214,7 +215,7 @@ gdk_x11_gl_context_flush_buffer (GdkGLContext *context)
     if (display_x11->has_glx_video_sync)
       glXGetVideoSyncSGI (&end_frame_counter);
 
-    if (!display_x11->has_glx_swap_interval)
+    if (context_x11->do_frame_sync && !display_x11->has_glx_swap_interval)
       {
         glFinish ();
 
@@ -757,6 +758,8 @@ gdk_x11_display_make_gl_context_current (GdkDisplay   *display,
 {
   GdkX11GLContext *context_x11 = GDK_X11_GL_CONTEXT (context);
   GLXDrawable drawable = None;
+  GdkScreen *screen;
+  gboolean do_frame_sync = FALSE;
 
   if (context_x11->glx_context == NULL)
     return FALSE;
@@ -778,14 +781,24 @@ gdk_x11_display_make_gl_context_current (GdkDisplay   *display,
         drawable = info->drawable;
       else
         drawable = gdk_x11_window_get_xid (window);
+
+
+      // If the WM is compositing there is no particular need to delay
+      // the swap when drawing on the offscreen, rendering to the screen
+      // happens later anyway, and its up to the compositor to sync that
+      // to the vblank.
+      screen = gdk_window_get_screen (window);
+      do_frame_sync = ! gdk_screen_is_composited (screen) && gdk_gl_context_get_swap_interval (context);
     }
 
   if (G_UNLIKELY (drawable == None))
     return FALSE;
 
-  if (drawable == context_x11->current_drawable)
+  if (drawable == context_x11->current_drawable && context_x11->do_frame_sync == do_frame_sync)
     return TRUE;
 
+  context_x11->do_frame_sync = do_frame_sync;
+
   GDK_NOTE (OPENGL,
             g_print ("Making GLX context current to drawable %lu (dummy: %s)\n",
                      (unsigned long) drawable,
@@ -799,7 +812,7 @@ gdk_x11_display_make_gl_context_current (GdkDisplay   *display,
 
   if (GDK_X11_DISPLAY (display)->has_glx_swap_interval)
     {
-      if (gdk_gl_context_get_swap_interval (context))
+      if (context_x11->do_frame_sync)
         glXSwapIntervalSGI (1);
       else
         glXSwapIntervalSGI (0);

gdk/x11/gdkglcontext-x11.h

@@ -51,6 +51,8 @@ struct _GdkX11GLContext
   GLXWindow dummy_glx_drawable;
 
   guint is_direct : 1;
+  guint do_frame_sync : 1;
+
 };
 
 struct _GdkX11GLContextClass