Spotted by Marcus Meissner.

Also the numbers we format are actually unsigned.

There's not much else we need to do for ARB_map_buffer_alignment. There's
still a pretty decent chance that we'll get lucky with alignment even on
drivers that don't support the extension, since mmap is going to return page
aligned memory. Tiny buffers might get packed together in a page though.

We don't need any specific render target here, since without a D3D device we
can't do any rendering anyway. We just want to avoid pointing to something
that was free'd.