Metal: Fix power function translation GLSL definition of pow(x, y) excludes negative x, so it must be translated to metal::powr, not metal::pow. Fixed: angleproject:8366 Change-Id: Idaa66da73a149119e2a74beddc9a49e187d3e7af Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4901995 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Kenneth Russell <kbr@chromium.org>