The sampleRateShading feature adds new builtins variables which have
to be supported: BuiltInSampleId and BuiltInSamplePosition. This cl
sets the values for these builtins properly and turns on per sample
rendering where either of these is persent in the shader.

Bug: b/171415086
Change-Id: I959e80722493d12213be349019a0cc3a108a7596
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51735Tested-by: Alexis Hétu <sugoi@google.com>
Commit-Queue: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>