fixed regression: foundation::Transform::identity() was returning a pair of zero matrices.

got bitten by the static data members initialization order.
minor tweaks.
added renderer::ParamArray::exist_path() and renderer::ParamArray::remove_path().
the texture store now has its own parameters.
allow to enable importance sampling in the light sampler.

the light sampler now has its own parameters.
importance sampling can be enabled via the "light_sampler.enable_importance_sampling" parameter.