FluidSimulation3D example: fix bug, and change header guards

[ttnghia]

Hi @mosra,

I did some changes for the FluidSimulation example:

• Fixed a bug: the particle color cannot be changed.
• Fixed a bug: the light direction cannot be changed.
• I changed header guards of the *.h files, adding example name. This is necessary if we have different examples having files with the same file names.
• Fixed some warnings.
• Improve menu.

Also, the link in the example page (https://doc.magnum.graphics/magnum/examples-fluidsimulation3d.html):

Full source code is linked below and also available in the magnum-examples GitHub repository.

is wrong.

[mosra]

Thank you! Much appreciated. I'll update the repo link in the docs (for the record, all that is in doc/fluidsimulation3d.dox in case you'd want to add things there).

[mosra]

Merged as 579a14c...db48e1d (without the #include changes), thanks!