glthread: use id 0 for internal buffer objects

glthread: use id 0 for internal buffer objects

-1 is an invalid buffer index which breaks app expectations, specifically
apitrace, which checks for return value of 0 from checking buffer bindings
to determine whether to inject user vertex buffer bindings and create functional
traces

this should fix capturing traces with drivers using glthread

fixes #8383

cc: mesa-stable

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22293>