Implement basic GLSL ES 3.0 texture sampling.

Bug 19331817

Change-Id: Ieb4581c62525112e0b76e94bcbbb8f9608c782ea
Reviewed-on: https://swiftshader-review.googlesource.com/2371
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>