gallivm/format: clamp SINT conversion rather than truncate.
KHR-GL32.packed_pixels.pbo_rectangle.r16i on zink on lavapipe ends up using a pbo that does an SINT image write. This was producing truncated rather than clamped values.
Fix the calculations for 8/16-bit signed ints to clamp not truncate.
Fixes: 13e5f331 ("gallivm/nir: fix image store conversions")
Edited by Dave Airlie