Add missing external texture type to the sampler uniforms.

Change-Id: I6954c992401bca495df3808603fba1bb5ecb8a60
Reviewed-on: https://swiftshader-review.googlesource.com/2740
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>