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>
2 files changed