Belt brim: move the apron bands with the purge plan's layer grid

The purge plan snaps every object onto one layer grid after the brim is built;
the per-layer brim bands follow their layers but the apron bands below the
first layer carry their own print_z and were left behind, which is why a brim
was refused next to a purge tower object. The shift now moves them too and the
combination is accepted again.

src/libslic3r/BeltPurge.cpp

@@ -322,6 +322,9 @@ void PrintObject::belt_shift_layer_grid(double delta)
         layer->print_z += delta;
     for (SupportLayer *layer : m_support_layers)
         layer->print_z += delta;
+    // The brim's apron bands below the first layer carry their own print_z.
+    for (BeltBrimBand &band : m_belt_brim_prologue)
+        band.print_z += delta;
     m_slicing_params.belt_floor_z_shift += delta;
 }
 

src/libslic3r/Print.cpp

@@ -1420,14 +1420,10 @@ StringObjectException Print::validate(std::vector<StringObjectException> *warnin
             return { L("Draft shield is not compatible with belt printer mode.") };
 
         // Belt brim spans many layers and owns the layers below the object, which
-        // neither the belt purge tower nor spiral vase can share (the purge plan moves
-        // objects onto a common layer grid after the brim bands are built). The prime
-        // tower setting alone is no obstacle: it stays on for any multi-filament
-        // project, and belt printers never print the classic tower.
+        // spiral vase cannot share. The prime tower setting is no obstacle: belt
+        // printers never print the classic tower, and the belt purge prism is an
+        // ordinary object that never takes a brim.
         if (this->has_belt_brim()) {
-            if (this->has_belt_purge_tower())
-                return { L("Brim is not compatible with the belt purge tower. "
-                           "Disable one of them.") };
             if (m_config.spiral_mode.value)
                 return { L("Brim is not compatible with spiral vase mode on a belt printer. "
                            "Disable one of them.") };

tests/fff_print/test_skirt_brim.cpp

@@ -930,9 +930,9 @@ TEST_CASE("Belt inner-only leading brim does not reject the prime tower or spira
         CHECK(print.validate().string.empty());
         CHECK_FALSE(gcode(print).empty());
     }
-    // An actual purge tower object is different: the purge plan moves objects onto a common
-    // layer grid after the brim bands are built, so the two are still refused together.
-    SECTION("a brim is refused next to a belt purge tower object") {
+    // A purge tower object is accepted too: the purge plan moves every object, apron
+    // bands included, onto one layer grid.
+    SECTION("a brim is accepted next to a belt purge tower object") {
         DynamicPrintConfig config = belt_brim_multifilament_config(2, {
             { "brim_type",               "outer_only" },
             { "brim_width",              4 },
@@ -945,12 +945,11 @@ TEST_CASE("Belt inner-only leading brim does not reject the prime tower or spira
         Print print;
         Model model;
         init_print({ cube(20), cube(20) }, print, model, config, &overrides);
-        CHECK(print.validate().string.empty());
-
         model.objects.back()->config.set_key_value("belt_purge_tower_object", new ConfigOptionBool(true));
         print.apply(model, config);
         REQUIRE(print.has_belt_purge_tower());
-        CHECK_FALSE(print.validate().string.empty());
+        CHECK(print.validate().string.empty());
+        CHECK_FALSE(gcode(print).empty());
     }
 }