refactor Windows classes to separate EGL/WGL/Backing Window

windows/wgl_context.h

@@ -0,0 +1,25 @@
+#ifndef _WGL_CONTEXT_H
+#define _WGL_CONTEXT_H
+
+#include <Windows.h>
+#include "opengl_texture_buffer.h"
+#include "flutter_render_context.h"
+#if WGL_USE_BACKING_WINDOW
+#include "backing_window.h"
+#endif
+namespace polyvox_filament {
+
+    class WGLContext : public FlutterRenderContext {
+    public:
+        WGLContext(flutter::PluginRegistrarWindows* pluginRegistrar, flutter::TextureRegistrar* textureRegistrar);
+        void CreateTexture(uint32_t width, uint32_t height, std::unique_ptr<flutter::MethodResult<flutter::EncodableValue>> result);
+        void* GetSharedContext();    
+    private:
+        HGLRC _context = NULL;
+        #if WGL_USE_BACKING_WINDOW
+        std::unique_ptr<BackingWindow> _backingWindow = nullptr;
+        #endif
+    };
+
+}
+#endif