feat(mosaic): option to maintain aspect ratio of image

[gianluparri03]

Describe your changes

Created a mosaic.Fit value that lets you specify only one size of the image to be rendered, while the other one will be calculated in order to maintain the aspect ratio. For example:

mosaic.New().Width(100).Height(mosaic.Fit) // Fits to width
mosaic.New().Height(100).Width(mosaic.Fit) // Fits to height

Related issue/discussion: #475

Checklist before requesting a review

• I have read CONTRIBUTING.md
• I have performed a self-review of my code

If this is a feature

• I have created a discussion
• A project maintainer has approved this feature request. Link to comment:
  feat(mosaic): option to maintain aspect ratio of image #475 (comment)