Add missing PCL_EXPORTS in pcl_visualizer.h

[CSBVision]

When using PCL in Visual C++ projects with enabled common language runtime support (/CLR option), including pcl_visualizer.h yields a link error LNK2001 since FPSCallback is used (cf. line 1986) while FPSCallback::Execute() is pure virtual and there is no other derived struct providing an implementation. Hence, supplying a default implementation fixes this link error.
Maybe see also this thread on stackoverflow where the same issue has already been discussed: https://stackoverflow.com/questions/26889614/error-lnk2001-when-including-pcl-visualizer-h

[taketwo]

The function FPSCallback::Execute is not pure virtual, and it is defined in the implementation file. Therefore, your proposed solution causes redefinition error (see Travis log).

As an alternative solution, can you instead try to add PCL_EXPORTS to the structure declaration, i.e.

struct PCL_EXPORTS FPSCallback : public vtkCommand

and see if it helps on Windows.

[CSBVision]

Thanks for your comment!
Indeed you are right, the function is implemented in the cpp file; Before I just checked the headers (no other references found) as well as the pcl visualization dll which does not export the FPSCallback. Adding the PCL_EXPORTS to the callback struct requires to recompile the whole visualization dll, I'll check this soon and might update the PR accordingly.

[taketwo]

If it turns out to work, please add PCL_EXPORTS to the other two sibling callback structures (i.e. ExitMainLoopTimerCallback, and ExitCallback). Thanks!

[CSBVision]

Sure, I added them and test it soon

[CSBVision]

Just finished the recompiling of PCL; the functions are exported by the dll right now and this fixes this issue. However, there seems to be a general problem in other files (for instance including <pcl/visualization/range_image_visualizer.h> yields link errors link errors as well). Maybe these are caused by some other missing PCL_EXPORTS, but I did not dig into this issue so far...

For the moment I updated pcl_visulizer.h in my PR, expecting a successful build :-)

[taketwo]

Thanks