Prevent JPX requests which can exceed server memory limits

[Keith Hughitt]

Currently it is possible to create requests for non-linked JPX files which fail because they exceed PHP's memory limit for the server. For example, the default memory limit for PHP on a clean Ubuntu 10.10 installation is 128M. Even if this is increased to 1G, it is possible for a JPX request to attempt to use over 4G.

While increasing available memory on servers where that is feasible is helpful, there should also be a mechanism in Helioviewer.org to avoid processing requests that are likely to exceed the server's own limits.