Explicitly cast int to size_t to avoid warnings
Move PrimeSense and Leap Motion supports to src/opt
Move the OpenGL codes to src/opt
Rename vis to tools as we will add other analysis tools there
Update README as we do not sort the photons anymore; update comments
Use the macro INTEROPERABLE to enable OpenGL interoperability
OpenGL works again
PrimeSense sensor support works again
Leap Motion support works again
Only allow PrimeSense and Leap Motion supports if OpenGL is enabled