render: introduce shared memory allocator

It allocates in local main memory via shm_open, and provides a FD
to allow sharing with other processes.

This is suitable for software rendering under the Wayland and X11
backends.

include/render/shm_allocator.h

@@ -0,0 +1,23 @@
+#ifndef RENDER_SHM_ALLOCATOR_H
+#define RENDER_SHM_ALLOCATOR_H
+
+#include <wlr/types/wlr_buffer.h>
+#include "render/allocator.h"
+
+struct wlr_shm_buffer {
+	struct wlr_buffer base;
+	struct wlr_shm_attributes shm;
+	void *data;
+	size_t size;
+};
+
+struct wlr_shm_allocator {
+	struct wlr_allocator base;
+};
+
+/**
+ * Creates a new shared memory allocator.
+ */
+struct wlr_shm_allocator *wlr_shm_allocator_create(void);
+
+#endif