Globalstorage and Anisotropic filtering

Adds a Global Storage of configuration values and global s if the GPU support it.

Usage :

- Call to InitGStore() at the begin of the program. Only one time necesary!
- Use Set(key, value) to remplace or create new values in the store
- Use Get(key, &value) to read values from the store. Return True if is found. If is not found, return false and not change the value

Example : 

    float val = 1.0f;  // Default value if it not found
    Sigma::GStore::Get("foo", &val);
    Bar(val); ...