CCEffectRefraction - Address feedback from Viktor

- Fix the blend of the main texture and the refracted environment texture. It was a multiply, now it's a
  premultiplied alpha blend.
- If the refracted texture coordinates would index outside the environment texture, just return the main
  texture.
CCEffectRefraction - Crash fix when sprite texture / sprite frame is nil
CCSprite - Set the sprite's texture rect from the normal map if there is no main texture.
Merge remote-tracking branch 'upstream/develop' into develop
CCEffectRefraction - Remove conditional based on the refracted texture coordinates

Instead use abs(), min(), and step() to arrive at a flag that is 1.0 if the refracted coords are in bounds and
0.0 if they are out. Thanks to Scott for pointing out this alternative solution.
Merge remote-tracking branch 'upstream/develop' into develop
Merge pull request #876 from thayerandrews/develop

CCEffectRefraction - Fix various issues uncovered by Viktor during testing