Allow filtering of upload resize dimensions

[DrewAPicture]

Background:

Plupload ships with a client-side upload resizer that was included with Core in 3.3 but had the action removed because of browser inconsistencies so I implemented it as ​a plugin at the time. Everything works just fine and dandy with zero problems except that the resize_height and resize_width vars are defaulted to the Large image sizes defined in media settings. It would be nice to be able to filter one or both of the dimensions outside the scope of the 'Large' size (when 'Original' becomes the same size as 'Large', you effectively lose an image size).

Patch attached.

You'll have to re-add this action to test:

add_action( 'post-upload-ui', 'media_upload_max_image_resize' );

Related: #19174, #19770

alternative to extract()

It occurs to me that using extract() in this instance might be irresponsible as we're outside the scope of a function. 21407-alternative.diff​ uses an array instead.