d3d11allocator: Work as if buffer pool when running on texture array mode

d3d11allocator: Work as if buffer pool when running on texture array mode

Because the size of texture array cannot be updated dynamically,
allocator should block the allocation request. This cannot be
done at buffer pool side if this d3d11 memory is shared among
multiple buffer objects. Note that setting NO_SHARE flag to
d3d11 memory is very inefficient. It would cause most likey
copy of the d3d11 texture.