Resolve read-after-write hazard in api samples [hpp_]oit_linked_lists.

[asuessenbach]

Description

The gather pass writes values into the fragment_buffer, while the combine pass reads from that buffer to sort the fragments.
Without the buffer barrier, that leads to a read-after-write hazard.

Build tested on Win11 with VS2022. Run tested on Win10 with NVidia GPU.

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[SaschaWillems]

Even though this works (due to the version baseline we use), shouldn't this be pipelineBarrier2KHR instead, as the sample explicitly requests the KHR sync2 extension? If this is supposed to use the non-KHR function, then there's no need to request the sync 2 KHR extension.