mesa: Add support for unpacking 32-bit integer formats to int spans.

mesa: Add support for unpacking 32-bit integer formats to int spans.

This is the inverse operation to _mesa_pack_rgba_span_int.  The 16-bit
code isn't done because of lack of testing and not being sure how sign
extension/clamping should be handled between, say, 16-bit int and
32-bit int or uint.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>