Fix off-by-one error in tiling that could create zero-width or
zero-height tiles.

On the Mac, this could cause a chain reaction that resulted in
`CFRelease()` of a null pointer if the window size happened to be a
precise multiple of the tile size.
Merge pull request #121 from servo/oboe

Fix off-by-one error in tiling that could create zero-width or zero-height tiles.