agg: Explicitly cast dimensions when creating Python buffer

On WASM, which is wholly 32-bit, casting unsigned int to signed long is
a narrowing conversion, which it seems to treat as an error. The Agg
buffer cannot be over `(1<<23)` in width or height, so this cast is
safe even with the smaller sizes.

Author: QuLogic

src/_backend_agg_wrapper.cpp

@@ -254,12 +254,12 @@ PYBIND11_MODULE(_backend_agg, m, py::mod_gil_not_used())
 
         .def_buffer([](RendererAgg *renderer) -> py::buffer_info {
             std::vector<py::ssize_t> shape {
-                renderer->get_height(),
-                renderer->get_width(),
+                static_cast<py::ssize_t>(renderer->get_height()),
+                static_cast<py::ssize_t>(renderer->get_width()),
                 4
             };
             std::vector<py::ssize_t> strides {
-                renderer->get_width() * 4,
+                static_cast<py::ssize_t>(renderer->get_width() * 4),
                 4,
                 1
             };