Fix file permissions of server generated files

Issue Summary

On servers with tight umasks files generated by server scripts are not automatically readable by the web server itself. This causes problems where an upload itself works fine, but generated image derivatives do not display. The simplest workaround for this is to simply chmod server generated files to a more open mask.

This is exactly what Drupal core does in several places (see color.module, for example in http://drupal.org/node/119196) - this is perhaps suboptimal security wise, but until there is a standard 'fix_file_perms' function in core it's what we have.

Here is a patch that does exactly what color module does with img_assist module generated files.