V211: an image rendition that keeps proportions can fix its height instead of its width.

A rendition that keeps proportions sets one side and lets the other follow each photo. Until now that side was always the width, which suits a vertical panorama (a 300 px wide column as tall as it needs to be) and not a horizontal one, which wants a fixed height and a width as long as it needs. fit_by says which side is fixed:

• width (the default, and what every existing rendition keeps);
• height: the rendition's height is the fixed side and its width is left empty.

A rendition that is a fixed box has both sides set and ignores the column. Only height is part of a rendition's spec hash, so no existing rendition is remade by this migration.

Locks

ALTER TABLE on the (tiny) renditions table; the default is a constant, so no row is rewritten.