- Added support for the sample operand in SamplerCore, which simply
  involves offsetting the buffer by the the sampleId * samplePitch.
- Also added a check so that sampleId is within the expected range
  and doesn't cause reading memory out of bounds.

Bug: b/135265531
Change-Id: Ie828d07db41d36befb34037156736a6576af0676
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38728Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>