[flutter] Added additional SkeletonDrawable.renderToXXX() methods for simple offscreen rendering.

spine-flutter/example/lib/dress_up.dart

@@ -67,7 +67,7 @@ class DressUpState extends State<DressUp> {
             ? const SizedBox()
             : Row(
                   children: [
-                    Container(width: thumbnailSize, child:
+                    SizedBox(width: thumbnailSize, child:
                       ListView(
                           children: _skinImages.keys.map((skinName) {
                             var rawImageData = _skinImages[skinName]!;
@@ -80,7 +80,7 @@ class DressUpState extends State<DressUp> {
                                 },
                                 child: _selectedSkins[skinName] == true
                                       ? box
-                                      : ColorFiltered(colorFilter: ColorFilter.mode(Colors.grey, painting.BlendMode.saturation,), child: box)
+                                      : ColorFiltered(colorFilter: const ColorFilter.mode(Colors.grey, painting.BlendMode.saturation,), child: box)
                             );
                           }).toList()
                       ),
@@ -97,5 +97,6 @@ class DressUpState extends State<DressUp> {
   void dispose() {
     super.dispose();
     _drawable?.dispose();
+    _customSkin?.dispose();
   }
 }

spine-flutter/lib/spine_flutter.dart

@@ -3308,7 +3308,7 @@ class SkeletonDrawable {
     }
   }
 
-  Future<RawImageData> renderToRawImageData(double width, double height) async {
+  PictureRecorder renderToPictureRecorder(double width, double height) {
     var bounds = skeleton.getBounds();
     var scale = 1 / (bounds.width > bounds.height ? bounds.width / width : bounds.height / height);
 
@@ -3324,6 +3324,17 @@ class SkeletonDrawable {
     canvas.translate(-(bounds.x + bounds.width / 2), -(bounds.y + bounds.height / 2));
     canvas.drawRect(const Rect.fromLTRB(-5, -5, 5, -5), paint..color = material.Colors.red);
     renderToCanvas(canvas);
+    return recorder;
+  }
+
+  Future<Uint8List> renderToPng(double width, double height) async {
+    final recorder = renderToPictureRecorder(width, height);
+    final image = await recorder.endRecording().toImage(width.toInt(), height.toInt());
+    return (await image.toByteData(format: ImageByteFormat.png))!.buffer.asUint8List();
+  }
+
+  Future<RawImageData> renderToRawImageData(double width, double height) async {
+    final recorder = renderToPictureRecorder(width, height);
     var rawImageData = (await (await recorder.endRecording().toImage(width.toInt(), height.toInt())).toByteData(format: ImageByteFormat.rawRgba))!.buffer.asUint8List();
     return RawImageData(rawImageData, width.toInt(), height.toInt());
   }