Refactor swizzle names and types.

Refactor swizzle names and types.

  - Plant a flag to say "pretend all the inputs are RGBA".
    This is how libpng thinks.
    This is the opposite of what the implementation had been doing,
    so I've rearranged everything to reflect the new orientation.

  - Rewrite the names to be less mysterious looking.  No more Xs.

  - Make the src type uniformly const void*, to allow for 888 (RGB) srcs.

This should be performance and pixel neutral.  (Please revert if it's not.)

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1626463002
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Review URL: https://codereview.chromium.org/1626463002