gallium/u_vbuf: use signed vertex buffers offsets for optimal uploads

Uploaded data must start at (stride * start), because we can't modify
start in all cases. If it's the first allocation, it's also the amount
of memory wasted. If the starting offset is larger than the size of
the upload buffer, the buffer is re-created, used for 1 upload, and then
thrown away. If the upload is small, most of the buffer space is unused
and wasted. Keep doing that and the OOM killer comes. It's actually
pretty quick.

With signed VB offsets, we can set min_out_offset = 0
in u_upload_alloc/u_upload_data.

This fixes OOM situations with SPECviewperf.

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