mesa: fix packing of float texels to GL_SHORT/GL_BYTE

mesa: fix packing of float texels to GL_SHORT/GL_BYTE

Previously, we would unpack the texels to floats using *_TO_FLOAT_TEX,
and then pack them into the desired format using FLOAT_TO_*. Unfortunately,
this isn't quite the inverse operation, and so some texel values would
end up off-by-one.

This fixes the GL_RGB8_SNORM and GL_RGB16_SNORM subcases in piglit's
arb_texture_view-format-consistency-get test on i965. The similar 1-, 2-
and 4-component cases already worked because they took the memcpy path
rather than repacking.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Eric Anholt <eric@anholt.net>