Add gl::ScopedColor default constructor

I think this is nice to have, and pretty intuitive, when you are setting multiple colors and drawing multiple things in the same drawing routine:

```
MyApp::draw()
{
  gl::ScopedModelMatrix scopedMat;
  gl::ScopedColor colorScope;

  gl::color( vec3( 1, 0, 0 ) );
  // draw something red..

  gl::color( vec3( 0, 0, 1 );
  // draw something blue..
}
```