This patch implements adjacency primitive types as new draw modes
on OpenGL back-ends.

This patch also implements validations on the compatibilities among
draw modes and geometry shader input primitive types.

BUG=angleproject:1941
TEST=dEQP-GLES31.functional.geometry_shading.input.*
     dEQP-GLES31.functional.geometry_shading.negative.*

Change-Id: I373ebfe88d7f50da3cc81adaf2d1b7f586b0932a
Reviewed-on: https://chromium-review.googlesource.com/954715
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>