Fix buffer protocol inheritance

Fixes #878.

tests/test_buffers.cpp

@@ -74,6 +74,11 @@ private:
     float *m_data;
 };
 
+class SquareMatrix : public Matrix {
+public:
+    SquareMatrix(ssize_t n) : Matrix(n, n) { }
+};
+
 struct PTMFBuffer {
     int32_t value = 0;
 
@@ -141,6 +146,10 @@ test_initializer buffers([](py::module &m) {
         })
         ;
 
+    // Derived classes inherit the buffer protocol and the buffer access function
+    py::class_<SquareMatrix, Matrix>(m, "SquareMatrix")
+        .def(py::init<ssize_t>());
+
     py::class_<PTMFBuffer>(m, "PTMFBuffer", py::buffer_protocol())
         .def(py::init<>())
         .def_readwrite("value", &PTMFBuffer::value)