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.cpp

@@ -2404,6 +2404,25 @@ void ImGuiWrapper::draw(
     }
 }
 
+void ImGuiWrapper::draw_gradient_ramp(ImDrawList *draw_list, const ImVec2 &top_left, const ImVec2 &bottom_right, const std::vector<wxColour> &ramp)
+{
+    if (draw_list == nullptr || ramp.empty() || bottom_right.x <= top_left.x || bottom_right.y <= top_left.y)
+        return;
+
+    const int    rows  = std::max(1, (int) std::lround(bottom_right.y - top_left.y));
+    const float  row_h = (bottom_right.y - top_left.y) / rows;
+    const size_t last  = ramp.size() - 1;
+    for (int r = 0; r < rows; ++r) {
+        // Row 0 is the top of the rect and so takes the ramp's last entry, the model's top.
+        const double    t = (rows > 1) ? (double) (rows - 1 - r) / (rows - 1) : 0.5;
+        const wxColour &c = ramp[(size_t) (t * last + 0.5)];
+        // The bottom row snaps to the rect's edge so rounding never leaves a sliver uncovered.
+        const float y0 = top_left.y + r * row_h;
+        const float y1 = (r + 1 == rows) ? bottom_right.y : top_left.y + (r + 1) * row_h;
+        draw_list->AddRectFilled({top_left.x, y0}, {bottom_right.x, y1}, IM_COL32(c.Red(), c.Green(), c.Blue(), c.Alpha()));
+    }
+}
+
 void ImGuiWrapper::draw_cross_hair(const ImVec2 &position, float radius, ImU32 color, int num_segments, float thickness) {
     auto draw_list = ImGui::GetOverlayDrawList();
     draw_list->AddCircle(position, radius, color, num_segments, thickness);