st/nine: Increase available GPU memory

st/nine: Increase available GPU memory

This patch caps to 4GB the limit of GPU memory accessible
only for 32bits build.
This would deserve some tests on windows, so we might change that
behaviour in the future. For example, it's possible that
GetAvailableTextureMem is capped to 4GB on 64bits build.

We cap to a bit less than 4GB, which might help
https://github.com/iXit/Mesa-3D/issues/323

In addition, increase from 80% to 95% the allocation limit above
which we fail allocating.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5015>