X11GLXContext.getPlatformExtensionsStringImpl(): Add comment from pull request regarding GLX extensions

https://github.com/sgothel/jogl/pull/107

Return either glXQueryClientString or glXQueryExtensionsString when getting the GLX extensions

ndjojo:
glXQueryExtensionsString will make a request for glXQueryServerString if needed and will append the necessary client-side extensions.
This doc, under the section "Using GLS Extensions", also suggests checking the glXQueryExtensionsString before using GLX extensions.

+++

aschleck:
For some more context this came up with the release of Mesa 20.3.0,
which has client support for GLX_EXT_swap_control but no server support.
The current JOGL behavior of appending the client extensions to the server extensions is incorrect.
They should instead be intersected (with client-only extensions then appended) as the doc Nicole linked above says,
which is precisely what glXQueryExtensionsString does.
With the current extension querying behavior JOGL thinks glXSwapIntervalEXT is available under Mesa/llvmpipe
even though it is not, causing a segfault at JOGL initialization time.

I originally filed this as a Mesa bug (https://gitlab.freedesktop.org/mesa/mesa/-/issues/4128)
along with some code that repro'd JOGL's checking behavior but it became apparent that Mesa is fine and the checking behavior is incorrect.

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