GL: Simplify members of ShaderGL. It does not need to store anything except the shader ID. Everything else can be queried from the context. Bug: angleproject:350779978 Change-Id: I202ae8e59e8c5b10ffb24f5454151b17818d400c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5672739 Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com> Reviewed-by: Roman Lavrov <romanl@google.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>