Add disabled support for GL_EXT_fog_coord. While it seems correct to me, it's

Add disabled support for GL_EXT_fog_coord.  While it seems correct to me, it's
not respecting the coords (or perhaps interpreting them differently?) in my
testing.  However, in the process it led to a fix of a secondary color handling
issue where it would be taken from the wrong offset, I believe, based off of
reading the r200 driver.  Also add a minor tweak to save time in the
fog-but-not-specular case.