Limit scaling of images. Fix #25

In that particular case in the issue, scale was ~ 0.55, so decided to limit minimum scale to 0.8 and maximum to 1.2.

Small trees still look quite small, but i guess it could be further improved by uploading a bigger texture.
![screenshot 2014-06-05 11 04 59](https://cloud.githubusercontent.com/assets/2392131/3185245/efb0a886-ec90-11e3-868d-adb80529d1c3.png)