ChromaStack: the colour model, and transmission distance per filament

A picture is reproduced on one face by stacking thin layers of a few
filaments, swapping filament at fixed layer boundaries, and varying how
high each pixel is built up. Filament is translucent, so a column that
stops part-way through a run shows a blend of that run's filament and what
is under it - which is where the colours come from, far more of them than
there are filaments loaded.

libslic3r/ChromaStack.hpp holds the whole model, free of the GUI. A stack
is a list of bands (a filament and its layer count, bottom-up) on a base
of solid backing. chroma_palette() turns it into the colour shown at every
column height, by Beer-Lambert absorption with each filament's
transmission distance; chroma_solve() maps every pixel to the height whose
colour is closest in CIELAB, optionally diffusing the rounding error, and
returns the 8-bit height map and the colours it predicts. Beer-Lambert is
the only model that stays consistent applied per layer, so the palette
does not depend on how the stack was cut into bands, and a test asserts
it.

chroma_layers_to_opaque() and chroma_distinct_levels() are what make a
stack's behaviour legible: how many layers a filament needs to hide what
is under it, and how many of a picture's layers change the colour at all.
chroma_ranges() turns the bands into object height ranges, which is how
the slicer is told to print a Z band in another filament.

Transmission distance is a property of the material, so it is a filament
preset option: filament_transmission_distance, with no effect on slicing.

src/libslic3r/Print.cpp

@@ -226,6 +226,7 @@ bool Print::invalidate_state_by_config_options(const ConfigOptionResolver & /* n
         "filament_diameter",
          "volumetric_speed_coefficients",
         "filament_density",
+        "filament_transmission_distance",
         "filament_cost",
         "filament_notes",
         "outer_wall_acceleration",