Shrink image sizes of attachments in Reply app. (#996)

During the investigation of
flutter/flutter#132690 , we discovered that
the Reply image previews are the fully size 2000x2000 images. The
decoding of these images is putting the device under heavy strain which
is making it hard to diagnose other performance issues.

lib/studies/reply/mail_card_preview.dart

@@ -1,4 +1,5 @@
 import 'package:animations/animations.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:gallery/layout/adaptive.dart';
 import 'package:gallery/studies/reply/colors.dart';
@@ -255,6 +256,16 @@ class _MailPreview extends StatelessWidget {
 class _PicturePreview extends StatelessWidget {
   const _PicturePreview();
 
+  bool _shouldShrinkImage() {
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.iOS:
+      case TargetPlatform.android:
+        return true;
+      default:
+        return false;
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     return SizedBox(
@@ -269,6 +280,7 @@ class _PicturePreview extends StatelessWidget {
               'reply/attachments/paris_${index + 1}.jpg',
               gaplessPlayback: true,
               package: 'flutter_gallery_assets',
+              cacheWidth: _shouldShrinkImage() ? 200 : null,
             ),
           );
         },

lib/studies/reply/mail_view_page.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:gallery/studies/reply/model/email_model.dart';
 import 'package:gallery/studies/reply/model/email_store.dart';
@@ -134,6 +135,16 @@ class _MailViewBody extends StatelessWidget {
 class _PictureGrid extends StatelessWidget {
   const _PictureGrid();
 
+  bool _shouldShrinkImage() {
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.iOS:
+      case TargetPlatform.android:
+        return true;
+      default:
+        return false;
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     return GridView.builder(
@@ -151,6 +162,7 @@ class _PictureGrid extends StatelessWidget {
           gaplessPlayback: true,
           package: 'flutter_gallery_assets',
           fit: BoxFit.fill,
+          cacheWidth: _shouldShrinkImage() ? 500 : null,
         );
       },
     );

lib/studies/shrine/app.dart

@@ -151,6 +151,7 @@ class _ShrineAppState extends State<ShrineApp>
 
     return ScopedModel<AppStateModel>(
       model: _model.value,
+      // ignore: deprecated_member_use
       child: WillPopScope(
         onWillPop: _onWillPop,
         child: MaterialApp(