Add idempotency guard to uiInit() on all platforms

[cody271]

No description provided.

[szanni]

Looks good in principle to me, however it actually breaks the uiInit(); uiUninit(); uiInit(); sequence on all platforms, not only darwin as highlighted in #118.

I believe setting the variable to false in uiUninit() would do the trick. In that case renaming the variable to something like uiprivInitialized would be more appropriate as well.

[e1732a364fed]

please check this issue before merge

#160