Vertical tiling / Row mode

[atagen]

Just documenting a discussion on preferences for implemention:

• ideally changes are constrained to src/layout/ with as little flow-on effect as possible
• single flag per output should be adequate
• no abstractions are necessary, columnar terminology and logic to be left intact with divergence in the code at a minimum (just for inputs, layout, etc)
• in all other respects, vertical tiles are effectively to be treated as "sideways columns"