code: static GVRMaterial getShadowMaterial(GVRContext ctx)
      {
          if (sShadowMaterial == null)
          {
              GVRShaderId depthShader = ctx.getShaderManager().getShaderType(GVRDepthShader.class);
              sShadowMaterial = new GVRMaterial(ctx, depthShader);
          }
          return sShadowMaterial;
      }
docs: Gets the shadow material used in constructing shadow maps.
  <p>
  Adds the shadow mapping depth shaders to the shader manager.
  There are two variants - one for skinned and one for non-skinned meshes.
  @return shadow map material