Metal: Fix ANGLEUniformBlock alignment To pass Metal validation, the length of ANGLEUniformBlock structure must be aligned to 16 bytes. Bug: angleproject:5505 Change-Id: I1e2becaac48739966afb44e087fc4a70f7b21d02 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3440067 Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org>