Examples: Fix GPGPU Birds warnings (See #5753)

- Fix warning X4121: gradient-based operations must be moved out of flow
  control to prevent divergence. Performance may improve by using a non-gradient operation
- This is seen in Firefox Stable (34) on Windows
- This fix seems to give small amount of fps boost
Examples: Make GPGPU Birds texture fetches more accurate
Merge pull request #5797 from zz85/fix_x4121_warnings

Fix x4121 warnings