Negative f for pow() in shaders

- Fixes the following errors, because of negative values in pow() function of shaders:
```
(71,64-116): warning X3571: pow(f, e) will not work for negative f, use abs(f) 
or conditionally handle negative values if you expect them
```
- Interestingly, hemiDotNormalHalfSky was always generating negative pow() error
- two shader samples are also fixed


related to #4859