Simple set of the camera index to allow the user to change it after the object has been initialized.

This adds a similar interface with pure-cpp and gpu versions.

Use predefined OpenCL function to convert integers to floating points.
This is more accurate than before as it enables:
1. saturate cast
2. customized rounding

Affected functions surf.ocl, pyrlk.ocl and hog.ocl are updated with the change.