[BUG] Non-Square Images Squished

### :information_source: Info
Version: `0.8.1`

### :speech_balloon: Description

It looks like non-square images are squished, rather than padded, when generating iOS favicons.

This also affects `web` favicon generation in https://github.com/fluttercommunity/flutter_launcher_icons/pull/155.

These images are from the `example` project directory.

#### Original (iOS)


#### Generated (iOS)


Is this expected behavior?

### :scroll: Pubspec.yaml
```yaml
name: example
description: A new Flutter project to quickly test flutter_launcher_icons.

version: 1.0.0+1

environment:
sdk: ">=2.7.0 <3.0.0"

dependencies:
flutter:
sdk: flutter
cupertino_icons: ^0.1.3
flutter_launcher_icons:
path: ../..

flutter_icons:
# image_path: "assets/images/icon-128x128.png"
image_path_android: "assets/images/icon-710x599-android.png"
image_path_ios: "assets/images/icon-710x599-ios.png"
image_path_web: "assets/images/icon-710x599-web.png"
android: true # can specify file name here e.g. "ic_launcher"
ios: true # can specify file name here e.g. "My-Launcher-Icon"
web: true # Can't specify file name here at present.
adaptive_icon_background: "assets/images/christmas-background.png" # only available for Android 8.0 devices and above
adaptive_icon_foreground: "assets/images/icon-foreground-432x432.png" # only available for Android 8.0 devices and above

dev_dependencies:
flutter_test:
sdk: flutter

flutter:
uses-material-design: true
```

Start with the example project's pubspec.yaml and its configured iOS and web image paths, then reproduce the generated assets using the non-square images described in the issue. Trace the image-generation entry points for iOS and web and compare their output with the originals. Done means non-square images preserve their aspect ratio by being padded rather than squished on both platforms.

Written by the indexing model from the issue text.