The sampler was simply not declared in that case, as if the uniform
was unused in the shader, so making sure structures containing
samplers call samplerRegister solves this issue. It was working
properly if the sampler was used anywhere else in the shader, but
failed when the only use in the shader was being passed as an
argument to a function through a containing structure.

Added a unit test since this isn't covered by dEQP.

Fixes 2 WebGL tests:
conformance/glsl/bugs/sampler-array-struct-function-arg.html
conformance/glsl/bugs/sampler-struct-function-arg.html

Change-Id: I81767d7c6415de7aefefecffcc66265d944a94ab
Reviewed-on: https://swiftshader-review.googlesource.com/18628Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>