Fixed #10043 -- widthratio tag now accepts a variable for the max_width argument.

The max_width argument now passes through FilterExpression which by side-affect
allows float values, but will be truncated.

Thanks obeattie and Eric Holscher for patches.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10352 bcc190cf-cafb-0310-a4f2-bffc1f526a37