Ensure StubBufferAllocator returns buffers with the properties requested,
and not the same old hardcoded constants. This will be required for testing
resizing (coming soon).
AFAIK, the reason for "geometry::Size{id, id}" was to uniquely identify
buffers in test cases. However that has since been superseded by Buffer::id().
Approved by Alan Griffiths, Alexandros Frantzis, PS Jenkins bot.