Vsync tweaks

Try to use adaptive vsync if available
Don't use vsync if unavailable

components/sdlutil/sdlgraphicswindow.cpp

@@ -113,7 +113,7 @@ void GraphicsWindowSDL2::init()
         return;
     }
 
-    SDL_GL_SetSwapInterval(_traits->vsync ? 1 : 0);
+    setSwapInterval(_traits->vsync);
 
     SDL_GL_MakeCurrent(oldWin, oldCtx);
 
@@ -194,11 +194,31 @@ void GraphicsWindowSDL2::setSyncToVBlank(bool on)
 
     SDL_GL_MakeCurrent(mWindow, mContext);
 
-    SDL_GL_SetSwapInterval(on ? 1 : 0);
+    setSwapInterval(on);
 
     SDL_GL_MakeCurrent(oldWin, oldCtx);
 }
 
+void GraphicsWindowSDL2::setSwapInterval(bool enable)
+{
+    if (enable)
+    {
+        if (SDL_GL_SetSwapInterval(-1) == -1)
+        {
+            OSG_NOTICE << "Adaptive vsync unsupported" << std::endl;
+            if (SDL_GL_SetSwapInterval(1) == -1)
+            {
+                OSG_NOTICE << "Vertical synchronization unsupported, disabling" << std::endl;
+                SDL_GL_SetSwapInterval(0);
+            }
+        }
+    }
+    else
+    {
+        SDL_GL_SetSwapInterval(0);
+    }
+}
+
 void GraphicsWindowSDL2::raiseWindow()
 {
     SDL_RaiseWindow(mWindow);

components/sdlutil/sdlgraphicswindow.hpp

@@ -80,6 +80,9 @@ public:
 
         SDL_Window *mWindow;
     };
+
+private:
+    void setSwapInterval(bool enable);
 };
 
 }