[Ogre1.x] Point and spot lights don't cast shadows

**Original report ([archived issue](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-rendering/issues/32)) by Louise Poubel (Bitbucket: [chapulina](https://bitbucket.org/%7B5cfa2075-477b-4ded-bdb9-8d2479544ec4%7D/), GitHub: [chapulina](https://github.com/chapulina)).**

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# Summary

All lights have the [Light::SetCastShadow](https://github.com/ignitionrobotics/ign-rendering/blob/6511cba2ed24608e83475b5adde766048f2be325/include/ignition/rendering/Light.hh#L106) function, but it only works if the light type is directional.

# Motivation

I noticed this while reviewing [this Ign-Gazebo PR](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-gazebo/pull-requests/70/add-support-for-lights/). The issue is analogous to [Gazebo issue 2083](https://bitbucket.org/osrf/gazebo/issues/2083/point-and-spot-lights-dont-cast-shadows).

# Describe alternatives you've considered

* Implement shadows for all light types
* Print a warning if the function is called for a light that doesn't support shadows

# Additional context

This blue spot light had shadows on: