Standalone implementation of gradient mapping with OpenGL shaders,
a modern version of paletted textures, to support nonlinear color gradients.

This removes the constraint mentioned in "Effective browsing of long audio recordings":
    "This decomposition constrains the image of $f$ to a straight line in RGB
    space (so hue, saturation, and value cannot be really independent),
    but it quadruples how much data can be browsed (see Sec.~\ref{sec:eightbit})."