Match mixed filament swatches to the editor's gradient preview

The sidebar Mixed Filament list, the extruder icons, the color painting
gizmo and the canvas filament bar now show the same bottom-to-top fade the
Edit Mixed Filament preview shows, custom gradient curves included, instead
of a horizontal fade between the two component colours. Ordinary and vendor
multi-colour filaments are drawn exactly as before.

src/slic3r/GUI/FilamentBitmapUtils.hpp

@@ -9,7 +9,7 @@
 
 // Orca: forward-declare so the header is self-contained outside libslic3r_gui's
 // force-included pch (the GUI test suite includes it directly).
-namespace Slic3r { class DynamicPrintConfig; }
+namespace Slic3r { class DynamicPrintConfig; struct GradientCurve; }
 
 namespace Slic3r { namespace GUI {
 
@@ -32,6 +32,33 @@ wxBitmap create_filament_bitmap(const std::vector<wxColour>& colors,
                               const wxSize& size,
                               bool force_gradient = false);
 
+// Blend colours at the given relative weights through blend_color_multi, so a measured
+// real-world mix is used where one exists instead of a plain channel lerp.
+wxColour blend_n_colors(const std::vector<wxColour>& cols, const std::vector<double>& weights);
+
+// Sample a gradient mixed filament the way the slicer builds it: t runs 0..1 over the
+// model's height, the curve gives the first component's ratio at t, and the two
+// components are blended at that ratio. Entry 0 is the bottom of the model, the last
+// entry its top. Blending goes through blend_n_colors, so measured mixes and the
+// reserved [kGradientMinRatio, kGradientMaxRatio] band are both respected — a plain
+// two-endpoint fade is neither.
+std::vector<wxColour> sample_gradient_ramp(const wxColour& first,
+                                           const wxColour& second,
+                                           const Slic3r::GradientCurve& curve,
+                                           int steps);
+
+// Same ramp for a project config slot, resolving components, colours and curve (or the
+// linear gradient_range fallback) from cfg. Empty unless the slot is a two-component
+// gradient mixed filament, which is what gates every caller to mixed slots only.
+// steps is the ramp's resolution; pass the destination's height in pixels.
+std::vector<wxColour> mixed_gradient_ramp(const Slic3r::DynamicPrintConfig& cfg, size_t slot, int steps);
+
+// Fill rect with a ramp, ramp.front() along the bottom edge.
+void fill_gradient_ramp_rect(wxDC& dc, const wxRect& rect, const std::vector<wxColour>& ramp);
+
+// Swatch bitmap for a gradient mixed filament, drawn bottom to top from the ramp.
+wxBitmap create_gradient_ramp_bitmap(const std::vector<wxColour>& ramp, const wxSize& size);
+
 // Recompute blended representative colors for mixed (virtual) filament slots.
 // Reads mixed-filament config keys from cfg and writes back into colors[i]
 // for every slot where filament_is_mixed[i] is true.