Avoid notices when array arguments are missing in WP_Image_Editor::multi_resize()

[dh-shredder]

WP_Image_Editor::multi_resize accepts an array of arrays, where each can contain:
'height'=>int, 'width'=>int, 'crop'=>bool

At the moment, all of these are required, under penalty of notice.

In other parts of our code, 'crop' is expected to default to false and not be required.

The attached patch makes 'height' and 'width' required (skipping the resizes gracefully if they are not provided), and automatically sets crop to false if it's not provided.

[dh-shredder]

edit: comment correction

[nacin]

In 24055:

Image editors: multi_resize() should require height and width. Crop is now optional and defaults to false. props DH-Shredder. fixes #23884.