Docs update
Update tools.rst
Add a #define for distance-based LODs in DrawInstanced
StateSetCache: additional API points for finer control over state sharing
Merge branch 'master' of github.com:gwaldron/osgearth
osgearth_mrt: added an RTT intersection demo.
Using different ramps for weather demo
Added support for RGBA color ramps
Merge branch 'master' of https://github.com/gwaldron/osgearth
Merge branch 'master' into topic-weather