Instead of using raw pointers, use `std::unique_ptr`.

Bug: b/126126820
Change-Id: Ia095f6aec1448c2153a0b0159f25a2946054e64b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43813Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>