Metal: Fix compute path for 16-bit PBO readbacks Added 16-bit normalized pixel formats to internal copy shaders. Fixed and simplified floatToNormalized usage for signed inputs. Bug: angleproject:352700368 Change-Id: Icbb79381991c6621004d53706b97662d33c25cd8 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5717929 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Quyen Le <lehoangquyen@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>