Switch to using staging buffer for vertex data

gpu.h

@@ -31,9 +31,9 @@ typedef struct {
 	VkFence in_flight;
 	VkSemaphore render_fin;
 	VkSemaphore img_avail;
+	size_t vertex_data_size;
 	VkBuffer vertex_buf;
 	VkDeviceMemory vertex_buf_mem;
-	size_t vertex_data_size;
 } gpu_ctx_t;
 
 typedef struct {