Derive Rasterizer from Function<>.

Bug 22652760

Change-Id: I1a8ff978fdc7ad795e469218a931523d624e7cde
Reviewed-on: https://swiftshader-review.googlesource.com/4546
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>