Use integers when positioning on the target canvas

This avoids blurred images after view center changes (dx, dy) and at fractional zoom levels (dw, dh).

Before:
![before](https://f.cloud.github.com/assets/211514/1342068/8c180edc-365a-11e3-87bf-6fc028ff7bea.png)

After:
![after](https://f.cloud.github.com/assets/211514/1342075/c12ad352-365a-11e3-81fa-e2f115569aa0.png)