types/wlr_buffer: introduce wlr_buffer_resource_interface

Adds `wlr_buffer_resource_interface` and `wlr_buffer_register_resource_interface`,
which allows a user to register a way to create a wlr_buffer from a specific
wl_resource.

include/wlr/types/wlr_buffer.h

@@ -69,6 +69,12 @@ struct wlr_buffer {
 	} events;
 };
 
+struct wlr_buffer_resource_interface {
+	const char *name;
+	bool (*is_instance)(struct wl_resource *resource);
+	struct wlr_buffer *(*from_resource)(struct wl_resource *resource);
+};
+
 /**
  * Initialize a buffer. This function should be called by producers. The
  * initialized buffer is referenced: once the producer is done with the buffer
@@ -112,6 +118,14 @@ bool wlr_buffer_get_dmabuf(struct wlr_buffer *buffer,
  */
 bool wlr_buffer_get_shm(struct wlr_buffer *buffer,
 	struct wlr_shm_attributes *attribs);
+/**
+ * Allows the registration of a wl_resource implementation.
+ *
+ * The matching function will be called for the wl_resource when creating a
+ * wlr_buffer from a wl_resource.
+ */
+void wlr_buffer_register_resource_interface(
+		const struct wlr_buffer_resource_interface *iface);
 /**
  * Transforms a wl_resource into a wlr_buffer and locks it. Once the caller is
  * done with the buffer, they must call wlr_buffer_unlock.

types/wlr_buffer.c

@@ -159,6 +159,28 @@ static bool buffer_is_shm_client_buffer(struct wlr_buffer *buffer);
 static struct wlr_shm_client_buffer *shm_client_buffer_from_buffer(
 	struct wlr_buffer *buffer);
 
+/* struct wlr_buffer_resource_interface */
+static struct wl_array buffer_resource_interfaces = {0};
+
+void wlr_buffer_register_resource_interface(
+		const struct wlr_buffer_resource_interface *iface) {
+	assert(iface);
+	assert(iface->is_instance);
+	assert(iface->from_resource);
+
+	const struct wlr_buffer_resource_interface **iface_ptr;
+	wl_array_for_each(iface_ptr, &buffer_resource_interfaces) {
+		if (*iface_ptr == iface) {
+			wlr_log(WLR_DEBUG, "wlr_resource_buffer_interface %s has already"
+					"been registered", iface->name);
+			return;
+		}
+	}
+
+	iface_ptr = wl_array_add(&buffer_resource_interfaces, sizeof(iface));
+	*iface_ptr = iface;
+}
+
 struct wlr_buffer *wlr_buffer_from_resource(struct wl_resource *resource) {
 	assert(resource && wlr_resource_is_buffer(resource));