buffer: permit writing into some abyss of tiles
Making the abyss geometry handling work with babl_process_rows is proving a bit tricky - the choice that skips the skipping at the start of a batch entering a tile seem to let all expected code paths work and no reported non-working code paths yet. This will also be a slight performance loss - but it might be that we seldom do writes that fall in the abyss anyways. Not writing into the abyss will permit better control over sliced rendering with multi-threading - and should be added back later.
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