swr: Add path to draw directly from client memory without copy.

swr: Add path to draw directly from client memory without copy.

If size of client memory copy is too large, don't copy. The draw will
access user-buffer directly and then block.  This is faster and more
efficient than queuing many large client draws.

Applications that still use large client arrays benefit from this.  VMD
is an example.

The threshold for this path defaults to 32KB.  This value can be
overridden by setting environment variable SWR_CLIENT_COPY_LIMIT.

v2: Use #define for default value, rather than hard-coded constant.

Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>