radv: Guard against misplaced RGP barrier markers.

radv: Guard against misplaced RGP barrier markers.

RGP will crash if we emit a layout transition marker outside a barrier.
If this happens, trigger an assertion if its enabled or silently discard
the marker otherwise to avoid traces that cannot be opened.

Also, guard against attempts to start barrier markers recursively, since
this will corrupt the internal start/end matching.

Closes: <https://gitlab.freedesktop.org/mesa/mesa/-/issues/7971>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21033>