pack: adjust clamping for int=>ubyte conversion
gles3conform expects than when converting from a signed
int to an unsigned byte, the output will be clamped at a
max of 0x7f. This impacts conversion from
int16_t => uint8_t and int32_t => uint8_t.
Signed-off-by:
Jordan Justen <jordan.l.justen@intel.com>