Shifted font vowels for Khmer

## Issue
Hello guys, first thanks for the awesome work. I have an issue with Khmer font, with or without a custom font (font is unicode and available on Google fonts `Hanuman-Regular.ttf`)

The top and bottom vowels are all shifted to the left as the screen shot below.


Hope you can help.

## Steps to Reproduce
```
ctx.fillText('Khmer: សូមអរគុណពីហ្គីកូខេមបូឌ', 10, 90)
```
Above, the fillText is displaying the vowels correctly (សូមអរគុណពីហ្គីកូខេមបូឌ) on github as utf-8, my file is encoded as utf-8, the output from fillText seems to have an issue aligning them.

## Your Environment
* node-canvas 2.x