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.
Merge pull request #5221 from nnethercote/dont-scale-single-char-text-divs

Don't scale single-char text divs.