Simplify transactions

Commit 4b8e3a885b makes it so only one
transaction is committed (ie. configures sent) at a time. This commit
removes the now-unnecessary code which was used to support concurrent
committed transactions.

* Instead of containers storing a list of instructions which they've
been sent, it now stores a single instruction.
* Containers now have an ntxnrefs property. Previously we knew how many
references there were by the length of the instruction list.
* Instructions no longer need a ready property. It was used to avoid
marking an instruction ready twice when they were in a list, but this is
now avoided because there is only one instruction and we nullify the
container->instruction pointer when it's ready.
* When a transaction applies, we no longer need to consider releasing
and resaving the surface, as we know there are no other committed
transactions.
* transaction_notify_view_ready has been renamed to
view_notify_view_ready_by_serial to make it consistent with
transaction_notify_view_ready_by_size.
* Out-of-memory checks have been added when creating transactions and
instructions.

include/sway/desktop/transaction.h

@@ -20,6 +20,8 @@
  * create and commits a transaction from the dirty containers.
  */
 
+struct sway_transaction_instruction;
+
 /**
  * Find all dirty containers, create and commit a transaction containing them,
  * and unmark them as dirty.
@@ -31,7 +33,8 @@ void transaction_commit_dirty(void);
  *
  * When all views in the transaction are ready, the layout will be applied.
  */
-void transaction_notify_view_ready(struct sway_view *view, uint32_t serial);
+void transaction_notify_view_ready_by_serial(struct sway_view *view,
+		uint32_t serial);
 
 /**
  * Notify the transaction system that a view is ready for the new layout, but

include/sway/tree/container.h

@@ -151,7 +151,12 @@ struct sway_container {
 	struct wlr_texture *title_urgent;
 	size_t title_height;
 
-	list_t *instructions; // struct sway_transaction_instruction *
+	// The number of transactions which reference this container.
+	size_t ntxnrefs;
+
+	// If this container is a view and is waiting for the client to respond to a
+	// configure then this will be populated, otherwise NULL.
+	struct sway_transaction_instruction *instruction;
 
 	bool destroying;