Short4 warnings fixed

Short4 warnings fixed

Switched the Short4 constructor from 'short' to 'unsigned short'
in order to accept hexadecimal values, which is generally how
Short4 values are initialized.

Change-Id: I89eb41bfd65f9071ba42dbb6a06d4349cbb89d67
Reviewed-on: https://swiftshader-review.googlesource.com/5701
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>