mesa/st, dri2, wgl, glx: Restore flush_objects interop backward compat

mesa/st, dri2, wgl, glx: Restore flush_objects interop backward compat

In commit 1396dc1c a new output field was added as a parameter, but this
is a problem since the signature of the function are not versionned.

The flush function didn't have a versionned output struct. So what I'm
proposing here is that if the version of the input argument is new enough
(bumped to 2 here), then we re-use the existing argument, which until now
was directly a pointer to GLsync, and instead use it as a pointer to a
versioned struct.

We're just changing one pointer type to another, so in C, this should
be fine AFAIK.

Fixes: 1396dc1c

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>