Right now we unconditionally flush because we don't otherwise handle host visible memory that isn't coherent. This adds some overhead, at some point we should make sure we track the different heap types and determine if the heap we've allocated from is already coherent.

Example of processing heap data:

https://github.com/flutter/engine/blob/main/impeller/renderer/backend/vulkan/capabilities_vk.cc#L374-L387

Where we would need to check:

https://github.com/flutter/engine/blob/main/impeller/renderer/backend/vulkan/device_buffer_vk.cc#L42-L45

That is, the VmaAllocationInfo info heapIndex will be the index of some specific heap, like https://vulkan.gpuinfo.org/displayreport.php?id=24254#memory . If we know this is coherent, avoid this call.