[graphite] Upload dither range uniform as a Half

Bug: b/256870077
Change-Id: Id7a5ec61c3a8106414430a3c9d83770cd2b01d95
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/661040
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
8 files changed