Adding more built-in function definitions

Defined more derivatives and texture functions
for glsl. By itself, this enables a few hundred
dEQP tests to pass for derivatives, which were
already implemented properly in some cases.

Change-Id: I84063205447ee4813409a3eb52aeb2ad79793a88
Reviewed-on: https://swiftshader-review.googlesource.com/3383
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>