Treat an image with no colour spread as flat, not as a ramp

The axis-coverage signal defaulted to 1, meaning "spans its range", which is
what a ramp measures. An image of a single colour has no axis to measure, so it
kept that default and stopped counting as flat - the most flat image there is.
It broke the slic3rutils test that feeds mix_targets() a uniform 8x8.

Zero is the right default: an unmeasurable axis leaves the flat-share test
deciding on its own, exactly as before this signal existed. That also covers an
image with too few pixels to fit an axis through.

src/libslic3r/TextureDisplacement.cpp

@@ -754,7 +754,9 @@ TextureDetail analyze_texture_detail(const TextureDisplacementLayer &layer)
 
         // Both signals, because either alone misreads a common case: the histogram calls a
         // two-colour gradient flat (measured 0.90 on a green-to-red ramp, and 0.96 on a painted
-        // model texture), while coverage alone would call a noisy two-colour logo a gradient.
+        // model texture), while coverage alone would call a noisy two-colour logo a gradient. An
+        // image whose axis could not be measured keeps coverage at 0, so the flat-share test decides
+        // alone - which is what an image of a single colour needs.
         out.flat_colors = out.flat_share >= 0.85f && out.axis_coverage < 0.25f;
     }
     std::lock_guard<std::mutex> lock(g_texture_detail_cache.mutex);

src/libslic3r/TextureDisplacement.hpp

@@ -508,7 +508,11 @@ struct TextureDetail
     // covers it end to end; a few flat colours sit in spikes with gaps between them. The histogram
     // above cannot tell those apart - a two-colour gradient occupies as few bins as the two colours
     // on their own - so both are needed before an image counts as flat.
-    float axis_coverage = 1.f;
+    //
+    // Zero when there was nothing to measure: an image of one colour, or too few pixels to fit an
+    // axis through. That is the right default, because it leaves the flat-share test deciding on its
+    // own, exactly as it did before this signal existed.
+    float axis_coverage = 0.f;
     bool  flat_colors = false;
 };
 TextureDetail analyze_texture_detail(const TextureDisplacementLayer &layer);