refactor Windows classes to separate EGL/WGL/Backing Window

windows/backing_window.h

@@ -0,0 +1,23 @@
+#ifndef _BACKING_WINDOW_H
+#define _BACKING_WINDOW_H
+
+#include <flutter/method_channel.h>
+#include <flutter/plugin_registrar_windows.h>
+#include <flutter/standard_method_codec.h>
+
+namespace polyvox_filament {
+
+class BackingWindow {
+    public:
+    BackingWindow(
+        flutter::PluginRegistrarWindows *pluginRegistrar,
+        int initialWidth, 
+        int initialHeight);
+    HWND GetHandle();
+    private:
+        HWND _windowHandle;
+        HWND _flutterRootWindow;
+}
+
+}
+#endif