CreateSampler should create a Sampler object by default

CreateSampler should create a Sampler object by default

It should be possible to query the default parameters
of a Sampler created with glGenSamplers, so
checkSamplerAllocation() was added to make sure a
created sampler was associated to a Sampler object
where appropriate.

I also cleaned up the API a little to share validation
functions.

Change-Id: I55d95c6663d41e2566b24eb76a431dd4b4da61e3
Reviewed-on: https://swiftshader-review.googlesource.com/3637
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>