Move PLS clear values back into context state The API that required packing raw data into a buffer was un-ergonomic for developers and difficult to implement for WebGL vendors. Bug: angleproject:7279 Change-Id: If7c98908c285462c5775e8e2d8811883be139f64 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3972376 Commit-Queue: Chris Dalton <chris@rive.app> Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>