spearmint: Change opengl2-readme.txt for Spearmint

Remove installing. Change running for Spearmint.
spearmint: Make r_dlightMode 3 work

Dlight culling must be called too many times, and loses some dlights,
when r_dlightMode 3 is set. Might cause issues in other cases too, so just
disable it for now. It's from ET and might not be that much benefit anyway.