Image link with relative mediaID generates wrong img src.
Assume you have an image file "wiki:dokuwiki-128.png" and you're editing
some page in a "wiki" namespace.
You can include that image with relative mediaID. The image is shown.
=> <img src="/dokuwiki/_media/wiki:dokuwiki-128.png" ...
However, when you try to include image link with relative mediaID, the
image is not shown. You can see that the URL of the image is incorrect.
=> <img src="/dokuwiki/_media/.:dokuwiki-128.png" ...
The reason of this bug is that the mediaID in image link is not processed
Attached patch (against devel 2009-10-10) would fix this problem.