Building on support for transform feedback capture of I/O block members,
this change optimizes capture of builtins other than gl_Position by
directly decorating members of gl_PerVertex.  Most importantly, this
allows us to reserve only one varying for transform feedback, instead of
as many builtins there could be (up to 4).

Bug: angleproject:3606
Change-Id: Ie0957802c657ed6c5aac538d92b860582ed6da45
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2601072Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>