render/vulkan: Recycle memory maps for stage spans

Remapping buffers on every use causes a lot of unwanted pagefaults.
Reuse the mapping to significantly speed up the memcpy.

include/render/vulkan.h

@@ -483,6 +483,7 @@ struct wlr_vk_shared_buffer {
 	VkBuffer buffer;
 	VkDeviceMemory memory;
 	VkDeviceSize buf_size;
+	void *cpu_mapping;
 	struct wl_array allocs; // struct wlr_vk_allocation
 };