PAN_MESA_DEBUG=overflow will place objects as close as possible to a
protected region at the end of the buffer, so that overflows segfault.
Caught the bugs in all four of the preceding commits.
v2: memset the BO to 0xbb to catch code expecting zeroed allocations.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17447>