initial work to split into dart_filament and flutter_filament

flutter_filament/lib/filament/flutter_filament_texture.dart

@@ -0,0 +1,9 @@
+class FlutterFilamentTexture {
+  final int width;
+  final int height;
+  final int flutterTextureId;
+  final int hardwareTextureId;
+
+  FlutterFilamentTexture(
+      this.flutterTextureId, this.hardwareTextureId, this.width, this.height);
+}