Improve golden API ergonomics, reduce need for pumper where possible (Resolves #41) (#42)

* Added @isGoldenScene tag to test runners
* Added support for different platforms per item in a Gallery.
* Screenshots now capture the whole widget tree and then extracts a desired region, so that the screenshots capture pixels in the app overlay.
* Added per golden size control

Author: matthew-carroll

lib/src/goldens/golden_camera.dart

@@ -16,42 +16,35 @@ class GoldenCamera {
   /// along with its [description].
   ///
   /// {@macro golden_image_bounds_default_finder}
+  ///
+  /// The photo captures a screenshot of the entire widget tree, and then extracts the pixels
+  /// within the [finder] region. This requires moving more pixel data, but this is done so
+  /// that the photo captures widgets that sit in the app overlay, such as the mobile drag
+  /// handles, magnifier, or popover toolbar for a text field.
   Future<void> takePhoto(String description, [Finder? finder]) async {
     finder = finder ?? find.byType(GoldenImageBounds);
 
     expect(finder, findsOne);
 
     final renderObject = finder.evaluate().first.findRenderObject();
     late final Image photo;
-    if (renderObject!.isRepaintBoundary) {
-      // The render object that we want to screenshot is already a repaint boundary,
-      // so we can directly request an image from it.
-      final repaintBoundary = finder.evaluate().first.renderObject! as RenderRepaintBoundary;
-      photo = await repaintBoundary.toImage(pixelRatio: 1.0);
-    } else {
-      // The render object that we want to screenshot is NOT a repaint boundary, so we need
-      // to screenshot the entire UI and then extract the region belonging to this widget.
-      if (renderObject is! RenderBox) {
-        throw Exception(
-          "Can't take screenshot because the root of the widget tree isn't a RenderBox. It's a ${renderObject.runtimeType}",
-        );
-      }
+    if (renderObject is! RenderBox) {
+      throw Exception(
+        "Can't take screenshot because the root of the widget tree isn't a RenderBox. It's a ${renderObject.runtimeType}",
+      );
+    }
 
-      // TODO: Try the following approach. It probably doesn't work because we're
-      //       using a TestRecordingPaintingContext with a non-test version of Canvas.
-      //       But maybe it will work out.
-      final pictureRecorder = PictureRecorder();
-      final canvas = Canvas(pictureRecorder);
-      final screenSize = renderObject.size;
+    final pictureRecorder = PictureRecorder();
+    final canvas = Canvas(pictureRecorder);
+    final screenSize = renderObject.size;
 
-      final paintingContext = TestRecordingPaintingContext(canvas);
-      renderObject.paint(paintingContext, Offset.zero);
+    final paintingContext = TestRecordingPaintingContext(canvas);
+    renderObject.paint(paintingContext, Offset.zero);
 
-      photo = await pictureRecorder.endRecording().toImage(
-            screenSize.width.round(),
-            screenSize.height.round(),
-          );
-    }
+    photo = await pictureRecorder.endRecording().toImage(
+          screenSize.width.round(),
+          screenSize.height.round(),
+        );
 
     _photos.add(
       GoldenPhoto(description, photo),