Read the Belt Tilt Only from the Belt G-code Header

Every printer's config block lists belt_slice_rotation_angle (default 45), so the processor marked all G-code as belt G-code: imported flat G-code got the belt view on a belt printer, and the belt-only Z handling in the processor ran for non-belt prints whose config block precedes the body. Take the angle only from outside the config block, where only the belt header writes it.

tests/fff_print/test_gcode_processor.cpp

@@ -0,0 +1,46 @@
+#include <catch2/catch_all.hpp>
+
+#include "libslic3r/libslic3r.h"
+#include "libslic3r/GCode/GCodeProcessor.hpp"
+#include "libslic3r/PrintConfig.hpp"
+
+#include "test_utils.hpp"
+
+#include <fstream>
+#include <string>
+
+using namespace Slic3r;
+using Catch::Matchers::WithinAbs;
+
+namespace {
+
+float processed_belt_tilt(const std::string &gcode)
+{
+    ScopedTemporaryFile temp(".gcode");
+    {
+        std::ofstream os(temp.string());
+        os << gcode;
+    }
+    GCodeProcessor proc;
+    proc.apply_config(FullPrintConfig{});
+    proc.process_file(temp.string());
+    return proc.get_result().belt_tilt_angle;
+}
+
+constexpr const char *body = "G1 X10 Y10 Z0.2 F3000\nG1 X20 Y10 E1 F1200\n";
+
+} // namespace
+
+TEST_CASE("The config block's belt angle does not mark G-code as belt G-code", "[GCodeProcessor][belt]")
+{
+    // Every printer's config block lists belt_slice_rotation_angle (default 45), belt or not.
+    const std::string gcode = std::string("; CONFIG_BLOCK_START\n; belt_printer = 0\n; belt_slice_rotation_angle = 45\n; CONFIG_BLOCK_END\n") + body;
+    CHECK_THAT(processed_belt_tilt(gcode), WithinAbs(0., 1e-6));
+}
+
+TEST_CASE("The belt header's angle marks G-code as belt G-code", "[GCodeProcessor][belt]")
+{
+    const std::string gcode = std::string("; belt_slice_rotation_angle = -45.0\n") + body +
+                              "; CONFIG_BLOCK_START\n; belt_printer = 1\n; belt_slice_rotation_angle = -45\n; CONFIG_BLOCK_END\n";
+    CHECK_THAT(processed_belt_tilt(gcode), WithinAbs(45., 1e-6));
+}