_fix_shader_crash

_fix_shader_crash

commit bc4d0fec4f791fb198ff316849aaf3faba24b45a
Author: Gaspard Thevenon <gaspard.thevenon@kitware.com>
Date:   Thu Feb 10 10:19:39 2022 +0100

    Fix shader crash in Multi Volume Rendering without GradientTF

    When using OpenGLGPUVolumeRayCastMapper with a MultiVolume,
    not specifying a gradient opacity TF produced an error in
    the composed shader (no argument would be given to functions which expected one),
    and nothing was rendered, although this TF is supposed to be optional.

    This commit fixes this by adding tests during the declarations of
    those functions inside the shader, and by changing their signatures as needed.
    Therefore, when no gradient opacity TF is given, no argument is expected
    and none is given.

Gbp-Pq: Name 120_fix_shader_crash.patch