Metal: Compile default shader source files separately. - Previously all default shader files were included in one file named master_source.metal. - Now they will be compiled separately and link together to generate binary shader. - Also generate a debug binary variant that will be used in debug build. Bug: angleproject:2634 Change-Id: Ic040835c4a729666e73afeba52f5be839b632396 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2281784 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>