Fixed regression for WGL full screen DWM hack.

The DwmFlush workaround for windowed windows on Nvidia caused jitter for
full screen windows.  The workaround is now only used for windowed mode
windows.

Fixes #519.

src/wgl_context.c

@@ -571,7 +571,7 @@ void _glfwPlatformMakeContextCurrent(_GLFWwindow* window)
 void _glfwPlatformSwapBuffers(_GLFWwindow* window)
 {
     // HACK: Use DwmFlush when desktop composition is enabled
-    if (_glfwIsCompositionEnabled())
+    if (_glfwIsCompositionEnabled() && !window->monitor)
     {
         int count = window->wgl.interval;
         while (count--)
@@ -589,7 +589,7 @@ void _glfwPlatformSwapInterval(int interval)
 
     // HACK: Disable WGL swap interval when desktop composition is enabled to
     //       avoid interfering with DWM vsync
-    if (_glfwIsCompositionEnabled())
+    if (_glfwIsCompositionEnabled() && !window->monitor)
         interval = 0;
 
     if (window->wgl.EXT_swap_control)