Create SamplerCore on demand.

Create SamplerCore on demand.

Previously we dynamically allocated an array of SamplerCores. This
isn't necessary and we can just create one as a temporary object where
used. This has the added advantage that we could have Reactor variables
as class members and keep them short-lived.

Change-Id: Ifb2e6edbf275aa793bd7880bd35384e16000007d
Reviewed-on: https://swiftshader-review.googlesource.com/13548
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>