initial work to split into dart_filament and flutter_filament

flutter_filament/windows/backing_window.h

@@ -0,0 +1,31 @@
+#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 flutter_filament {
+
+class BackingWindow {
+    public:
+    BackingWindow(
+        flutter::PluginRegistrarWindows *pluginRegistrar,
+        int width, 
+        int height,
+        int left,
+        int top);
+    HWND GetHandle();
+    void Resize(int width, int height, int left, int top);
+    private:
+        HWND _windowHandle;
+        HWND _flutterRootWindow;
+        HWND _flutterViewWindow;
+        uint32_t _width = 0;
+        uint32_t _height = 0;
+        uint32_t _left = 0;
+        uint32_t _top = 0;
+};
+
+}
+#endif