Correctly handle inactive varyings of structure type when collecting
transform feedback varyings. Previously, all varyings of structure type
were marked as active by mistake.

Bug: angleproject:5557
Test: TransformFeedbackTest.InactiveStructureVarying*
Change-Id: Ica1cbce5c551cdbb97999eeac59d09a961937615
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2770680
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>