Material properties added

Lighting model now takes into account material properties. These are
used to simulate different types of materials, such as clay (very
matte), metal (shiny), and anything in between. Find these tweaks in
SceneRenderer.Render(). The line

m_constantBufferData.material = Material(1.0f, 1.0f, 0.6f, 1.0f);

Allows you to change the values. First argument is the ambient light
intensity, second is diffuse light intensity, third is specular
intensity, and the fourth is the "shininess" value, or the size of the
specular highlights (decrease this to get a larger highlight)