CJK characters cause cursor position to be incorrect in Safari 7

[jmcphers]

When a line contains CJK characters, the cursor position after the characters is wrong in Safari 7. Here's an example: after four CJK characters, the cursor blinks in the middle of the M instead of to its left:

The problem appears to be that in Safari, monospace font pixel widths can be fractional. The ace_cjk characters have widths applied that attempt to make them exactly twice as wide as regular characters:

However, it appears that Safari is rounding these widths down to 14px, and the 0.42px error accumulates over the line and eventually causes the cursor position to be significantly off.

(all screenshots are from the Ace Kitchen Sink)

[nightwing]

Merging this issue into #460