Generate unique HLSL texture function names for external textures. 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/388551 Reviewed-by: Corentin Wallez <cwallez@chromium.org>