FIX macos: Use the agg buffer_rgba rather than private attribute

[greglucas]

PR summary

I'm getting copy_agg_buffer failed and no plots to show up with the macosx backend.

The _renderer attribute has changed with the pybind11 update and we aren't able to access it as a buffer anymore. We can use the buffer_rgba method to get the direct memoryview of the buffer instead.

git bisect shows me it is this commit: 96dd843
ping @QuLogic

[anntzer]

(Seems close enough to #18993 which did the same for tk.)

[QuLogic]

Looking at the implementation of RendererAgg, it's:

    def buffer_rgba(self):
        return memoryview(self._renderer)

so this looks like a reasonable change. I'm a bit uncertain why PyObject_GetBuffer is not sufficient with pybind11's version of buffers, but we can investigate that for later.

[QuLogic]

So in the original implementation:

matplotlib/src/_backend_agg_wrapper.cpp

Lines 415 to 437 in ac268ec

	int PyRendererAgg_get_buffer(PyRendererAgg *self, Py_buffer *buf, int flags)
	{
	Py_INCREF(self);
	buf->obj = (PyObject *)self;
	buf->buf = self->x->pixBuffer;
	buf->len = (Py_ssize_t)self->x->get_width() * (Py_ssize_t)self->x->get_height() * 4;
	buf->readonly = 0;
	buf->format = (char *)"B";
	buf->ndim = 3;
	self->shape[0] = self->x->get_height();
	self->shape[1] = self->x->get_width();
	self->shape[2] = 4;
	buf->shape = self->shape;
	self->strides[0] = self->x->get_width() * 4;
	self->strides[1] = 4;
	self->strides[2] = 1;
	buf->strides = self->strides;
	buf->suboffsets = NULL;
	buf->itemsize = 1;
	buf->internal = NULL;
	return 1;
	}

we always filled out all fields regardless of flags. The macosx backend asks for a contiguous read-only buffer:

matplotlib/src/_macosx.m

Line 1167 in fbf1611

if (PyObject_GetBuffer(renderer, buffer, PyBUF_CONTIG_RO) == -1) {

which is an alias for PyBUF_ND.

In pybind11 however, it does attempt to respect flags, but unfortunately the implementation was wrong as it only filled out ndim and shape if the flags were PyBUF_STRIDES (a higher condition that PyBUF_ND). As it defaulted to ndim=1 otherwise, macosx would then fail the shape check:

matplotlib/src/_macosx.m

Line 1172 in fbf1611

if (buffer->ndim != 3 || buffer->shape[2] != 4) {

I've opened pybind/pybind11#5407 to fix pybind11.