mesa/pack: don't apply transfer operations to integer format buffers. (v3)

mesa/pack: don't apply transfer operations to integer format buffers. (v3)

The EXT_texture_integer issues says:

Should pixel transfer operations be defined for the integer pixel
path?

RESOLVED: No.  Fragment shaders can achieve similar results
with more flexibility.  There is no need to aggrandize this
legacy mechanism.

v2: fix comments, fix unpack paths, use same comment/code
v3: fix last comment

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Brian Paul <brianp@vmware.com>