initial work to split into dart_filament and flutter_filament

flutter_filament/lib/filament/widgets/transparent_filament_widget.dart

@@ -0,0 +1,16 @@
+import 'package:flutter/material.dart';
+
+class TransparencyPainter extends CustomPainter {
+  @override
+  void paint(Canvas canvas, Size size) {
+    canvas.drawRect(
+      Rect.fromLTWH(0, 0, size.width, size.height),
+      Paint()
+        ..blendMode = BlendMode.clear
+        ..color = const Color(0x00000000),
+    );
+  }
+
+  @override
+  bool shouldRepaint(covariant CustomPainter oldDelegate) => false;
+}