[patch] Just make samplegl3 working on Linux

[ebachard]

Hi,

In my quest to understand more about the imgui story, I found your project. Thanks a lot to guys like you who shared such precious code. If you are interested, I'll attach a patch allowing to build the samplegl3 test on Linux.

I tried to not break the other OS.

That's not a big deal, but if you are interested, I'm happy to share that :-) Linux_samplegl3.diff.txt

-- ericb