correct stride in matrix example and test

This also matches the Eigen example for the row-major case.

This also enhances one of the tests to trigger a failure (and fixes it in the PR).  (This isn't really a flaw in pybind itself, but rather fixes wrong code in the test code and docs).

docs/advanced/pycpp/numpy.rst

@@ -41,7 +41,7 @@ completely avoid copy operations with Python expressions like
                 py::format_descriptor<float>::format(), /* Python struct-style format descriptor */
                 2,                                      /* Number of dimensions */
                 { m.rows(), m.cols() },                 /* Buffer dimensions */
-                { sizeof(float) * m.rows(),             /* Strides (in bytes) for each index */
+                { sizeof(float) * m.cols(),             /* Strides (in bytes) for each index */
                   sizeof(float) }
             );
         });

tests/test_buffers.cpp

@@ -107,7 +107,7 @@ TEST_SUBMODULE(buffers, m) {
             return py::buffer_info(
                 m.data(),                               /* Pointer to buffer */
                 { m.rows(), m.cols() },                 /* Buffer dimensions */
-                { sizeof(float) * size_t(m.rows()),     /* Strides (in bytes) for each index */
+                { sizeof(float) * size_t(m.cols()),     /* Strides (in bytes) for each index */
                   sizeof(float) }
             );
         })

tests/test_buffers.py

@@ -36,17 +36,21 @@ def test_from_python():
 # https://bitbucket.org/pypy/pypy/issues/2444
 @pytest.unsupported_on_pypy
 def test_to_python():
-    mat = m.Matrix(5, 5)
+    mat = m.Matrix(5, 4)
-    assert memoryview(mat).shape == (5, 5)
+    assert memoryview(mat).shape == (5, 4)
 
     assert mat[2, 3] == 0
-    mat[2, 3] = 4
+    mat[2, 3] = 4.0
+    mat[3, 2] = 7.0
     assert mat[2, 3] == 4
+    assert mat[3, 2] == 7
+    assert struct.unpack_from('f', mat, (3 * 4 + 2) * 4) == (7, )
+    assert struct.unpack_from('f', mat, (2 * 4 + 3) * 4) == (4, )
 
     mat2 = np.array(mat, copy=False)
-    assert mat2.shape == (5, 5)
+    assert mat2.shape == (5, 4)
-    assert abs(mat2).sum() == 4
+    assert abs(mat2).sum() == 11
-    assert mat2[2, 3] == 4
+    assert mat2[2, 3] == 4 and mat2[3, 2] == 7
     mat2[2, 3] = 5
     assert mat2[2, 3] == 5
 
@@ -58,7 +62,7 @@ def test_to_python():
     del mat2  # holds a mat reference
     pytest.gc_collect()
     assert cstats.alive() == 0
-    assert cstats.values() == ["5x5 matrix"]
+    assert cstats.values() == ["5x4 matrix"]
     assert cstats.copy_constructions == 0
     # assert cstats.move_constructions >= 0  # Don't invoke any
     assert cstats.copy_assignments == 0