Define GLSL ES 3.0 sampler types.

Bug 19331817

Change-Id: I7f7f842d01069abda953a5fe3eb8572f5dc985a9
Reviewed-on: https://swiftshader-review.googlesource.com/2370
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>