gl_Position is excepted from contributing to MAX_VARYING_VECTORS, which
is taken into account when calculating this limit based on Vulkan limits
(which don't have such an exception).  This exception was mistakenly
also being applied to other limits such as MAX_VERTEX_OUTPUT_COMPONENTS.

Bug: angleproject:5916
Change-Id: I394761efa02d3093741d2bff21023daf997e5e69
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2869145Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>