ShaderCore const inputs

Made all inputs const in ShaderCore.

Change-Id: I56792cea564f40e6f5b096349447b22439606e43
Reviewed-on: https://swiftshader-review.googlesource.com/3410
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>