[sldev] Code cleanup / performance patches

[Dirk Moerenhout]

VWR-1612 and VWR-1613

The performance impact of these combined shouldn't exceed 2 to 3% at
best but it does also help keeping code clean and logical.

The addTextureStats (VWR-1612) optimises towards how the function is
called in over 90% of the cases.

The lltree/lloctree (VWR-1613) stuff is just cleaning up the overuse
of virtual as this currently stops the compiler (at least in VS2005)
from inlining.

Dirk aka Blakar Ogre