This patch adds the support of struct arrays as valid
geometry shader user-defined inputs.

Struct arrays are accepted as geometry shader inputs to
match the vertex shader outputs that are also declared
as structs.

BUG=angleproject:1941
TEST=angle_unittests

Change-Id: I0b5d545b10e9dda576a1c96d7c93ec2450611e9e
Reviewed-on: https://chromium-review.googlesource.com/823622
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>