weston_buffer: Add mode to weston_buffer_reference

Add a mode argument to weston_buffer_reference which indicates whether a
buffer's storage may/will be accessed, or whether the underlying storage
will no longer be accessed, e.g. because it has been copied. This will
be used to retain a pointer to the weston_buffer whilst being able to
send a release event to the client.

Signed-off-by: Daniel Stone <daniels@collabora.com>

include/libweston/libweston.h

@@ -1201,8 +1201,15 @@ struct weston_buffer {
 	uint64_t format_modifier;
 };
 
+enum weston_buffer_reference_type {
+	BUFFER_REF_NONE,
+	BUFFER_MAY_BE_ACCESSED,
+	BUFFER_WILL_NOT_BE_ACCESSED,
+};
+
 struct weston_buffer_reference {
 	struct weston_buffer *buffer;
+	enum weston_buffer_reference_type type;
 };
 
 struct weston_buffer_viewport {