This patch implements a new query target GL_PRIMITIVES_GENERATED_EXT
for querying the number of primitives generated by geometry shader.

BUG=angleproject:1941
TEST=dEQP-GLES31.functional.geometry_shading.query.primitives_generated*

Change-Id: Icdc0d2d8be4ff4d589972eada2fa1f917d7bae4c
Reviewed-on: https://chromium-review.googlesource.com/963793Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>