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>
(cherry picked from commit
dadbec1e90415f0744eb91e684bf9d7496f474c0)