As ANGLE doesn't redeclare gl_PerVertex, glslang always defines it with:

    gl_Position
    gl_PointSize
    gl_ClipDistance
    gl_CullDistance

The unused members here contribute to varying component limits.  The
last two are unlikely to be used, and the second member is rarely used
as well.

This change keeps it simple and strips the trailing inactive members,
which for all intents and purposes accurately minimizes this struct.

Bug: angleproject:5405
Change-Id: I59c22af4988a3da7b1e428913d0ea13be9031cea
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2562754
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>