Space mouse support about glvis

[v-dobrev]

GLVis uses SDL2, so the question of adding support for space mouse is if SDL2 detects the device and then what type of SDL_* events it generates, so we can process them in various places, e.g. here:

glvis/lib/sdl.cpp

Line 362 in c2e621c

void SdlWindow::mainIter()

here:

glvis/lib/sdl_main.cpp

Line 188 in c2e621c

void SdlMainThread::DispatchSDLEvents()

and probably other places.

One can probably add potential event candidates to the big switch in the method SdlMainThread::DispatchSDLEvents() with a print statement to see what events are generated by the device. The potential candidates are most likely the following SDL events: https://github.com/libsdl-org/SDL/blob/9237f8edc16c88028a6de5514196fb798c4a0f5c/include/SDL_events.h#L113-L133