Metal: Assert while using gl_VertexID as ivec Metal [vertex_id] is uint while gl_VertexID is int. Replacing gl_VertexID with gl_VertexIDMetal without rewrites of the expressions would cause invalid expressions. Fix by casting uint kgl_VertexIDMetal to int kgl_VertexID instead of replacing variable during compile. Bug: angleproject:8597 Change-Id: I76acdb2a0ab5982aa05181175925b3359068e901 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5376498 Reviewed-by: Alexey Knyazev <lexa.knyazev@gmail.com> Auto-Submit: Kimmo Kinnunen <kkinnunen@apple.com> Commit-Queue: Kenneth Russell <kbr@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org>