Add limit to Flickr photo size

[crazyscot]

In my Flickr account I have some oversized panoramas (e.g. 21034x3217 pixels). I have been experiencing browser crashes on my Raspi from MagicMirror and suspect these large images are part of the puzzle.

There's no point in burning the CPU, bandwidth and memory to retrieve and decode an image much larger than the attached display. This PR adds a limit configuration; we automatically select the largest available variant, subject to the limit.

[kolbyjack]

The config already has maxWidth and maxHeight items for this purpose, can you make the flickr source honor those instead of introducing new fields?

[crazyscot]

Thanks, I had missed those.