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/ImGuiWrapper.hpp

@@ -3,10 +3,12 @@
 
 #include <string>
 #include <map>
+#include <vector>
 #include <cstdlib>
 
 #include <imgui/imgui.h>
 
+#include <wx/colour.h>
 #include <wx/string.h>
 
 #include "libslic3r/Point.hpp"
@@ -299,6 +301,20 @@ public:
                                 int           num_segments = 0,
                                 float         thickness    = 4.f);
 
+    /// <summary>
+    /// Fill a rect with a filament gradient ramp, one band per pixel row, ramp.front() along
+    /// the bottom edge. Bands rather than one interpolated rect, because the ramp follows the
+    /// slot's gradient curve and ImGui's corner interpolation could only draw a straight fade.
+    /// </summary>
+    /// <param name="draw_list">Define where to draw it</param>
+    /// <param name="top_left">Upper left corner of the rect</param>
+    /// <param name="bottom_right">Lower right corner of the rect</param>
+    /// <param name="ramp">Colours printed, bottom of the model first</param>
+    static void draw_gradient_ramp(ImDrawList *          draw_list,
+                                   const ImVec2 &        top_left,
+                                   const ImVec2 &        bottom_right,
+                                   const std::vector<wxColour> &ramp);
+
     /// <summary>
     /// Check that font ranges contain all chars in string
     /// (rendered Unicodes are stored in GlyphRanges)