Use fActualProc to choose fFastProc or fSlowProc in SkSwizzler

(1) We will now (correctly) choose fFastProc if the client
    calls setSampleX(1).
(2) The logic to choose between fFastProc and fSlowProc
    is now performed once, rather than every time we call
    swizzle().
(3) This will be easier to extend when we add
    fFastSampleProcs for different sample sizes.

This comes from Leon's suggestion in:
https://codereview.chromium.org/1582083005

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1616753003

Review URL: https://codereview.chromium.org/1616753003
2 files changed