vulkan: flush allocations if needed

There is no guarantee that the created buffer is actually host coherent.
If it is not we have to manually flush it.

src/modules/graphics/vulkan/StreamBuffer.h

@@ -62,6 +62,7 @@ private:
 	VmaAllocationInfo allocInfo;
 	VkBuffer buffer = VK_NULL_HANDLE;
 	int frameIndex = 0;
+	bool coherent;
 
 };