Fix creation of AGG images bigger than 1024**3 pixels

Various buffer size calculations were overflowing, because C is awesome.
Simple reproduction case:

```python
import matplotlib
matplotlib.use('agg')
import matplotlib.pyplot as plt
fig, ax = plt.subplots(figsize=(463, 232), dpi=100)
fig.savefig('big.png')
```

(A size of 462x232 is just *under* the gigapixel threshold.)

Author: pkgw

src/_backend_agg.cpp

@@ -29,7 +29,7 @@ RendererAgg::RendererAgg(unsigned int width, unsigned int height, double dpi)
     : width(width),
       height(height),
       dpi(dpi),
-      NUMBYTES(width * height * 4),
+      NUMBYTES((size_t) width * (size_t) height * 4),
       pixBuffer(NULL),
       renderingBuffer(),
       alphaBuffer(NULL),

src/_backend_agg_wrapper.cpp

@@ -95,7 +95,7 @@ int PyBufferRegion_get_buffer(PyBufferRegion *self, Py_buffer *buf, int flags)
     Py_INCREF(self);
     buf->obj = (PyObject *)self;
     buf->buf = self->x->get_data();
-    buf->len = self->x->get_width() * self->x->get_height() * 4;
+    buf->len = (long) self->x->get_width() * (long) self->x->get_height() * 4;
     buf->readonly = 0;
     buf->format = (char *)"B";
     buf->ndim = 3;
@@ -531,7 +531,7 @@ 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 = self->x->get_width() * self->x->get_height() * 4;
+    buf->len = (long) self->x->get_width() * (long) self->x->get_height() * 4;
     buf->readonly = 0;
     buf->format = (char *)"B";
     buf->ndim = 3;