Drop off  scissor test enable on OnInitThread()

This fixes the render-to-texture sample background 

Before (with glEnable(GL_SCISSOR_TEST))
![1](https://cloud.githubusercontent.com/assets/3000282/3282186/2d92f466-f4dd-11e3-8140-ddc9809dc508.jpg)

-After (without glEnable(GL_SCISSOR_TEST))
![2](https://cloud.githubusercontent.com/assets/3000282/3282189/5de1cee4-f4dd-11e3-92d3-b63308063c1b.jpg)