Refactor input layout & vertex signatures. Always size input layouts to gl::MAX_VERTEX_ATTRIBS, and use '1' bits to signal where we have a GPU conversion. The simplification allows us to more cleanly match the vertex executable signatures and makes our default VertexExecutable hit much more often. BUG=510151 TEST=angle_end2end_tests,Canary WebGL, manual testing with Chromium Change-Id: I5009323c4e7e208e7a2595be46658c344517a4ff Reviewed-on: https://chromium-review.googlesource.com/290740 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>