Insert gap pixels only between chips unless module gaps are set

An unset GapPixels::module_gaps defaulted to the chip gap, so every
module boundary received two extra rows or columns. Two stacked
Jungfrau modules read as 1030x1030 instead of 1030x1028. Unset now
means no pixels between modules; only the chip boundaries inside a
module receive a gap.

Also drop the explicit lambda capture in split_edge_counts that clang
reported as unused for floating point pixels.

python/tests/test_RawFile.py

@@ -527,8 +527,9 @@ def test_raw_file_gap_pixels(tmp_path, layout):
     gaps = GapPixels(fill_value=5)
     with RawFile(master_path, gap_pixels=gaps) as reader:
         assert reader.gap_pixels == gaps
-        assert (reader.rows(), reader.cols()) == (512 * layout_rows + 2 * (layout_rows * 2 - 1),
-                                                  1024 * layout_cols + 2 * (layout_cols * 4 - 1))
+        # one chip boundary per module row, three per module column and no
+        # gap between modules
+        assert (reader.rows(), reader.cols()) == ((512 + 2) * layout_rows, (1024 + 6) * layout_cols)
         header, image = reader.read_frame()
         assert header.size == layout_rows * layout_cols
         assert (image == insert_gap_pixels(expected, DetectorType.Jungfrau, gaps)).all()