Because 2D and external textures have the same HLSL texture type, they were
generating texture functions with the same name.  This causes conflicts when
both 2D and external textures are used in the same shader.

BUG=angleproject:1534
BUG=645532

Change-Id: I4b324014b7d9b4851d358730cf4e31fc8461584c
Reviewed-on: https://chromium-review.googlesource.com/388551Reviewed-by: Corentin Wallez <cwallez@chromium.org>