Fixed #8113. Made `get_width_height` a `GoogleZoom` method that takes the...

Fixed #8113.  Made `get_width_height` a `GoogleZoom` method that takes the extent instead of an envelope so it may handle Point geometries.  Thanks to Santiago Aguiar for the bug report.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8428 bcc190cf-cafb-0310-a4f2-bffc1f526a37