backend: automatically create allocator

Introduce a new backend_get_allocator function that automatically
creates an allocator for the backend if the backend has a renderer.

backend/backend.c

@@ -19,6 +19,7 @@
 #include <wlr/util/log.h>
 #include "backend/backend.h"
 #include "backend/multi.h"
+#include "render/allocator.h"
 #include "util/signal.h"
 
 #if WLR_HAS_X11_BACKEND
@@ -36,6 +37,7 @@ void wlr_backend_init(struct wlr_backend *backend,
 
 void wlr_backend_finish(struct wlr_backend *backend) {
 	wlr_signal_emit_safe(&backend->events.destroy, backend);
+	wlr_allocator_destroy(backend->allocator);
 	wlr_renderer_destroy(backend->renderer);
 }
 
@@ -116,6 +118,23 @@ uint32_t backend_get_buffer_caps(struct wlr_backend *backend) {
 	return backend->impl->get_buffer_caps(backend);
 }
 
+struct wlr_allocator *backend_get_allocator(struct wlr_backend *backend) {
+	if (backend->allocator != NULL) {
+		return backend->allocator;
+	}
+
+	struct wlr_renderer *renderer = wlr_backend_get_renderer(backend);
+	if (renderer == NULL) {
+		return NULL;
+	}
+
+	backend->allocator = wlr_allocator_autocreate(backend, renderer);
+	if (backend->allocator == NULL) {
+		wlr_log(WLR_ERROR, "Failed to create backend allocator");
+	}
+	return backend->allocator;
+}
+
 static size_t parse_outputs_env(const char *name) {
 	const char *outputs_str = getenv(name);
 	if (outputs_str == NULL) {

include/backend/backend.h

@@ -10,4 +10,10 @@
  */
 uint32_t backend_get_buffer_caps(struct wlr_backend *backend);
 
+/**
+ * Get the backend's allocator. Automatically creates the allocator if
+ * necessary.
+ */
+struct wlr_allocator *backend_get_allocator(struct wlr_backend *backend);
+
 #endif

include/wlr/backend.h

@@ -29,6 +29,7 @@ struct wlr_backend {
 	// Private state
 
 	struct wlr_renderer *renderer;
+	struct wlr_allocator *allocator;
 };
 
 /**