Don't scale single-char text divs.

This change makes scrolling noticeably smoother on files with many
single-char text divs, such as the one in #1045. The trade-off is that
the visual appearance of text selection in such documents is slightly
worse, because more text divs overlap.

This change also uses `scaleX(N)` instead of `scale(N, 1)`. This might
be marginally more efficient in terms of JS string concatenation.

Here's a screenshot showing text selection and search selection prior to my change:

![genre-old](https://cloud.githubusercontent.com/assets/1940286/3991352/7d8334b8-28e0-11e4-8468-ea48138581f3.png)

Here's the equivalent screenshot after my change:

![genre-new](https://cloud.githubusercontent.com/assets/1940286/3991358/9364fc44-28e0-11e4-9802-500333a8c359.png)

The old code gave very little overlap, but some gaps. The new code gives more overlap, but no gaps. Neither of them look great!