Watermark image is not scaled correctly

# Description

When applying a watermark `sourceFile` to an image the scaling is not correctly applied to the varying output sizes (see below).


## Reproduction Steps

Steps to reproduce the behavior:
1. Configure a set of icons as per: https://mobilebuildtools.com/images/configuring-images/#watermarking-images
2. Build and check the `obj` folders
3. See error

## Expected Behavior

The watermark image should be stretched/shrunk to correctly fit the whole output icon.

## Actual Behavior

As can be seen in the screenshot the smaller icon sizes result in a smaller watermark.

## Environment

- OS: Windows 10
- IDE: VS2019
- BuildTools Version: 2.0.245
- IDE Version: 16.10.2
- Build Target Framework: netstandard2.0, Xamarin.iOS, Xamarin.Android

## Reproduction App

Please include a sample app that reproduces the issue and/or a binlog. Failure to reproduce either of these may lead to the issue being closed or ignored.