Tiled Gallery: Remove 5 px width adjustment.

The grouper used in computing the layout for rectangular galleries initially had a 5 pixel adjustment in it's content width: `$content_width - 5`, which was documented to be there for broken themes and rounding errors. After some discussion in #443, @ethitter concluded that it should be safe to remove the adjustment.
Merge pull request #519 from rase-/remove/content-width-adjustment

Tiled Gallery: Remove 5 px width adjustment as its stated reason for being cannot be reproduced.