Metal: Refactor textureGradOffset wrappers * Removed variadic macros. * Removed invalid overloads. * Updated emitter to not output unused variants. * Added MSL compile guards to not fail shader compilation on macOS 10.15. Bug: angleproject:368059227 Change-Id: I47e0c04176bd9f4b6e6abcd266ae4c673135c93b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5893503 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>