make SkSamplingOptions fields const

This lets us trust the invariants that the constructors set up,
like if fUseCubic, then fMipmap == SkMipmapMode::kNone.

Gotta be slightly sneaky and implement operator= ourselves
with placement new.

Change-Id: I043cd56cbf9f1e2fa64ab585268b2799df65c31f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/340179
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed