Belt brim: lattice lines closer to the belt than the band fraction move uphill

With a first layer of about 0.28 mm or more at 45 degrees (or a shallower
belt) the brim band is wider than one bead and its lines go on the nominal
lattice. A lattice line could land where the belt is almost at the band's
print_z; its flow was clamped to half a layer while the nozzle sat nearly on
the belt. Such a line now moves uphill to the 0.75 fraction the single-line
case uses, and a line that lands on the previous one is skipped.

Ported from the Unlayered fork (patch 0007 of its belt port series, found
there by fuzzing first layer heights). The fork's companion fix, restricting
the brim filament to those the writer was handed (0008), is not needed here:
ToolOrdering registers the brim filament on every band's layer, so the writer
always has it. A test pins that with every object a flush target.

tests/fff_print/test_skirt_brim.cpp

@@ -15,6 +15,7 @@
 #include <limits>
 #include <map>
 #include <set>
+#include <sstream>
 #include <string>
 
 #include "test_helpers.hpp" // get access to init_print, etc
@@ -1195,3 +1196,60 @@ TEST_CASE("Belt brim coexists with support material", "[SkirtBrim][belt]")
     REQUIRE(! gc.empty());
     CHECK(role_passes(gc, "brim") > 0);
 }
+
+// With a 0.3 mm first layer at 45 degrees the brim band on the belt is wider than one bead,
+// so its lines go on the nominal lattice instead of at a fixed fraction of the band. A
+// lattice line can then land where the belt is almost at the band's print_z; it must be
+// moved uphill to the same 0.75 fraction the single-line case uses, not laid scraping the
+// belt with its flow clamped to half a layer.
+TEST_CASE("Belt brim lattice lines keep their clearance above the belt", "[SkirtBrim][belt]")
+{
+    DynamicPrintConfig config = belt_brim_config();
+    config.set_deserialize_strict({
+        { "layer_height",               0.3 },
+        { "initial_layer_print_height", 0.3 },
+        { "brim_type",                  "outer_only" },
+        { "brim_width",                 4 },
+        { "brim_object_gap",            0 },
+    });
+    const std::string gcode = slice({ cube(20) }, config);
+
+    // Heights of the brim extrusions, from the ;HEIGHT: tags inside ;TYPE:Brim sections.
+    std::vector<double> brim_heights;
+    bool                in_brim = false;
+    std::istringstream  lines(gcode);
+    for (std::string line; std::getline(lines, line); ) {
+        if (boost::starts_with(line, ";TYPE:"))
+            in_brim = boost::starts_with(line, ";TYPE:Brim");
+        else if (in_brim && boost::starts_with(line, ";HEIGHT:"))
+            brim_heights.push_back(std::stod(line.substr(8)));
+    }
+    REQUIRE(! brim_heights.empty());
+    for (const double h : brim_heights) {
+        CHECK(h >= 0.75 * 0.3 - 1e-3);
+        CHECK(h <= 0.3 + 1e-3);
+    }
+}
+
+// The brim prints in the object's outer wall filament even when every extrusion of the object
+// is offered to purging (flush_into_objects): the tool ordering registers the brim filament
+// itself, so the writer always knows it.
+TEST_CASE("Belt brim slices when every object is a flush target", "[SkirtBrim][belt]")
+{
+    DynamicPrintConfig config = belt_brim_multifilament_config(2, {
+        { "brim_type",          "outer_only" },
+        { "brim_width",         4 },
+        { "brim_object_gap",    0 },
+        { "flush_into_objects", 1 },
+        { "flush_into_infill",  1 },
+    });
+    const std::vector<std::vector<Slic3r::ConfigBase::SetDeserializeItem>> overrides {
+        { { "extruder", 1 } }, { { "extruder", 2 } },
+    };
+    Print print;
+    Model model;
+    init_print({ cube(20), cube(20) }, print, model, config, &overrides);
+    REQUIRE(print.validate().string.empty());
+    const std::string out = gcode(print);
+    CHECK(out.find(";TYPE:Brim") != std::string::npos);
+}