When argument pointers pushed into an vector and the pointers point to
address on stack, we need to make sure they are valid until kernels are
successfully flushed onto the queue.

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.