Fixed crash in track info screen after changing resolution.

Widgets after changing resolution are deleted, but TrackInfoScreen object and m_screenshot variable still exsist. Therefore check if it's not NULL isn't enough.
Added a config file which can be used to disable certain graphical
options dependeing on card, os, driver version.
Do not use 0,0,0 as default sun position when unspecified, as this will give unpredictable results when normalized
Ignore billboard text nodes in physics, fixes #1666
Use CS for gaussian6
Try to fix crash at startup.
Merge branch 'master' of github.com:supertuxkart/stk-code
Prevent online user names starting with 0-9 (for #1471).
Try graphical restrictions on nvidia old drivers