fix Sub-Sector-Image in Web-Example

[SimonWaldherr]

the web-example shows the full mandelbrot fractal, if you click on areas you should get a zoomed version of this sector.

[lummie]

There does seem to be some inconsistencies when clicking and drilling on the image, sometimes it works and sometimes it doesn't I'll have a look.

Design question: Is there a reason you write the images to disk cache rather than directly to the response? Cache is starting to get quite big and the browser can cache the images anyway.

[SimonWaldherr]

@lummie would be great if you find an elegant solution/fix

there is no "real" reason, i just wanted to save them for debugging reasons and to reduce cpu load