fixed SPIR-V int->uint conversion Bug: skia: Change-Id: I31c035ac5839d60521ee9d8f73c879776ef5d22d Reviewed-on: https://skia-review.googlesource.com/24621 Commit-Queue: Ethan Nicholas <ethannicholas@google.com> Reviewed-by: Brian Salomon <bsalomon@google.com>