Fixes for screen capture related issues.

MAGN-553 Screen capture only work within original canvas borders
MAGN-742 Screen capture should not capture the grid

Improved screen capture to 150 DPI for better image quality. This value could be modify in Configurations.cs.

![image](https://f.cloud.github.com/assets/5276243/1711044/12dc9d30-6143-11e3-87d2-7072330a6ce1.png)