mesa: When unpacking signed integer pixel data, don't clamp to 0.

mesa: When unpacking signed integer pixel data, don't clamp to 0.

In the core, we always treat spans of int/uint data as uint, so this
extract function was truncating storage of integer pixel data to a n
int texture to (0, max_int) instead of (min_int, max_int).  There is
probably missing code for handling truncation on conversion between
pixel formats, still, but this does improve things.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: Brian Paul <brianp@vmware.com>