Acknowledge existing 'srcset' values when providing suggested 'srcset' values

[Daniel Bachhuber]

Given the following example:

It would be helpful if the bookmarklet acknowledged the srcset values that already existed, when providing suggested srcset values. Of the suggested values, I think I only need to create something in the ~450 range?

[Martin Auswöger]

Implemented in 9d1c6d8