Metal: Refactor texture wrappers * Removed variadic macros. * Removed invalid overloads. * Added MSL compile guards to not fail shader compilation on macOS 10.15. Bug: angleproject:368059227 Change-Id: I5376b410670a50cf700f1c33823f43c89c10b876 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5893780 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>