fixed bug with arial.ttf

The arial.ttf we using now is a merge of DejaVu Sans 2.30 and Droid Sans Fallback 1.00. There is an issue of the font file: can't show latin languages charaters right when size too small. This because the origin DejaVu Sans font's em=2048 and the merge file change it to 256.

Rebuild the merge file and use em=2048 solve the problem.
And update to use version 2.34 of DejaVu Sans.

build script:
```
# Font merge script
# usage: fontforge -script merge.pe

Open("DroidSansFallback.ttf")
SelectAll()
ScaleToEm(2048)
Generate("temp.ttf")
Close()

Open("DejaVuSans.ttf")
SelectAll()
#ScaleToEm(2048)

MergeFonts("temp.ttf")
Generate("arial.ttf")
Close()
```