Allow default image file format in layout defaults

[fluffy-critter]

Expected Behavior

Having the app configured with e.g.

{
    'layout': {
        'format': 'webp'
    }
}

should have all images rendered as webp by default.

Current Behavior

The format layout options do not appear to be propagated, and the default is still the source file's native format.

Possible Solution

Steps to Reproduce (for bugs)

Context