Use DwmFlush when DWM is enabled.

Use DwmFlush instead of WGL_EXT_swap_control when desktop compositing is
enabled, to avoid the jitter of DWM and WGL vsync fighting.

Fixes #516.

src/wgl_context.h

@@ -44,6 +44,7 @@ typedef struct _GLFWcontextWGL
 {
     HDC       dc;              // Private GDI device context
     HGLRC     context;         // Permanent rendering context
+    int       interval;
 
     // WGL extensions (context specific)
     PFNWGLSWAPINTERVALEXTPROC           SwapIntervalEXT;