A combined image sampler may need several descriptors in a descriptor
set. We are not currently checking how many descriptors are required,
nor is it presumably guaranteed that such multi-descriptor allocation
will not fail due to fragmentation.
If the pool free counter is not zero, try to allocate but continue with
the next pool and fall back to creating a new pool if the allocation
failed.
Fixes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/4010