Merge pull request #39 from kuu/master

- Fixed a text folding/overwrap issue.
[Issue]
  Text is overwraped by other text.
[Cause]
  Some contents use two EditTexts with different colors overlaying one over another.
  Such contents use space (U+3000) characters for offsetting,
  i.e. it inserts multiple U+3000 between words so that it can switch the color of each word.
  However, CanvasRenderingContext2D#fillText/measureText() replace all the space characters with ASCII space (U+0020) characters.
  And it causes the reduced space between each word.
[Fix]
  Replacing a single U+3000 with four U+0020 in advance eliminates the issue.
Improved the previous change with more precise calculation.