To make it easier to branch on the different texture
fetching options, a new TextureFunction class is
introduced here, which performs the string comparisons
and identifies the different options.

I also had to add a 5th argument for textureGradOffset
and textureProjGradOffset.

I added function stubs (with the UNIMPLEMENTED markers)
for all new texture functions.

Change-Id: I58cde91a2bacb0012bdc34ec85b0befa19a85326
Reviewed-on: https://swiftshader-review.googlesource.com/4116Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>