This patch adds the entry point and related validation for
FramebufferTextureEXT defined in OpenGL ES 3.1 extension
EXT_geometry_shader.

BUG=angleproject:1941
TEST=angle_end2end_tests

Change-Id: Id6804e0b3971f52273562ce1a325d8377926a558
Reviewed-on: https://chromium-review.googlesource.com/1069842
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>