Remove call to glFlush() before swap_buffers

We may not want to call `swap_buffers` in the same thread as the
current thread, so we're moving the call to `glFlush` out of the scope
of glutin.

src/win32/ffi.rs

@@ -720,9 +720,6 @@ extern "system" {
     // http://msdn.microsoft.com/en-us/library/windows/desktop/ms633519(v=vs.85).aspx
     pub fn GetWindowRect(hWnd: HWND, lpRect: *mut RECT) -> BOOL;
 
-    //
-    pub fn glFlush();
-
     // http://msdn.microsoft.com/en-us/library/windows/desktop/ms684175(v=vs.85).aspx
     pub fn LoadLibraryW(lpFileName: LPCWSTR) -> HMODULE;
 

src/win32/mod.rs

@@ -228,9 +228,6 @@ impl Window {
     /// See the docs in the crate root file.
     pub fn swap_buffers(&self) {
         unsafe {
-            // calling glFlush is necessary on Windows 8
-            ffi::glFlush();
-
             ffi::SwapBuffers(self.hdc);
         }
     }