[PATCH] Performance optimization using get_field()

WFS views waste CPU time, rendering each field twice: first in views_plugin_style::render_fields(), and after that in views_plugin_style_wfs::map_rows(). Attached is a patch (against CVS HEAD) which omits the 2nd rendering, instead using results from the 1st (available via views_plugin_style::get_field() function).

I am not a Views expert, so perhaps I got something wrong. But it works for me, and makes my view run 40% faster.