docs: static GVRMaterial getShadowMaterial(GVRContext ctx)
      {
          if (sShadowMaterial == null)
          {
              GVRShaderId depthShader = ctx.getShaderManager().getShaderType(GVRDepthShader.class);
              sShadowMaterial = new GVRMaterial(ctx, depthShader);
          }
          return sShadowMaterial;
      }