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 (more information in this doc, under the section "Extension availability").

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.