fix leak of temporary GL context on windows

Author: micahrj

src/gl/win.rs

@@ -192,6 +192,7 @@ impl GlContext {
         };
 
         wglMakeCurrent(hdc_tmp, std::ptr::null_mut());
+        wglDeleteContext(hglrc_tmp);
         ReleaseDC(hwnd_tmp, hdc_tmp);
         UnregisterClassW(class as *const WCHAR, hinstance);
         DestroyWindow(hwnd_tmp);