We were casting stencil ref value instead of clamping it as the spec
requires. Fixing that which allows a number of new dEQP tests to pass.

Bug: angleproject:3244
Bug: angleproject:3245
Bug: angleproject:3247
Bug: angleproject:3248
Change-Id: I59242d59982b990e82395f5a1e77b9a54254dc2b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1541751
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>