SphereRenderer does not utilize connected transfer function

[Gegell]

Describe the bug
The SphereRenderer does not seem to bind the transfer function correctly to a texture. The fallback to the gray (/linear ramp) function still generates the expected result, however upon connecting the TF the spheres all switch to the same color (black).

To Reproduce
See attached Minimal Reproducible Example below.

Expected behavior
The particles should obtain coloring according to their scalar value and the connected transfer function, and not only default to 0.

Additional context
I did a bit of digging and happened upon following TODO in the SphereRenderer.cpp, which I suspect is responsible for the unexpected bahaviour:

bool SphereRenderer::enableTransferFunctionTexture(std::shared_ptr<glowl::GLSLProgram> prgm) {

    mmstd_gl::CallGetTransferFunctionGL* cgtf = this->get_tf_slot_.CallAs<mmstd_gl::CallGetTransferFunctionGL>();
    if ((cgtf != nullptr) && (*cgtf)(0)) {
        // TODO: how to do with unique?
        //cgtf->BindConvenience(prgm, GL_TEXTURE0, 0);
    } else {
        glEnable(GL_TEXTURE_1D);
        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_1D, this->grey_tf_);
        glUniform1i(prgm->getUniformLocation("tfTexture"), 0);
        glUniform2fv(prgm->getUniformLocation("tfRange"), 1, static_cast<GLfloat*>(this->range_.data()));
    }
    return true;
}

And here a minimal working example: sphere_renderer_ignores_tf.zip

Screenshots
Here the comparison as seen in the sample file with the transfer function connected (scalar value of particles is simply their y-value):

And without the connection: