A bit more refactoring to expose the begin/end transactional nature.

vk_engine.h

@@ -7,6 +7,11 @@
 #include <vk_descriptors.h>
 #include "vk_gui.h"
 
+struct Transaction {
+  VkCommandBuffer cmd;
+  uint32_t swapchainImageIndex = 0;
+};
+
 struct DeletionQueue {
   std::deque<std::function<void()>> deletors;
 
@@ -42,7 +47,7 @@ public:
   VkPipeline _gradientPipeline;
   VkPipelineLayout _gradientPipelineLayout;
 
-  DescriptorAllocator globalDescriptorAllocator;
+  DescriptorAllocator _globalDescriptorAllocator;
   VkDescriptorSet _drawImageDescriptors;
   VkDescriptorSetLayout _drawImageDescriptorLayout;
 
@@ -108,6 +113,9 @@ public:
 
   void immediate_submit(std::function<void(VkCommandBuffer cmd)>&& function);
 
+  Transaction begin_transaction();
+  void commit_transaction(Transaction& cmd);
+
 private:
   void init_vulkan();
   void init_swapchain();