Write transparent pixels more often (SkGifCodec)

Writing transparent pixels is faster than the alternative, and we can
skip clearing the frame to transparent. We'll still clear if the image
is incomplete.

I ran

  ./out/Release/nanobench --images <images> --samples 100 --sourceType image --simpleCodec -v

over the GIFs we have on our bots, and found an average ~13% speedup.
Raw data is on sheet 2 of
https://docs.google.com/spreadsheets/d/19V-t9BfbFw5eiwBTKA1qOBkZbchjlTC5EIz6HFy-6RI/
(the sheet is named WriteTransparentPixels).
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2436183002

Review-Url: https://codereview.chromium.org/2436183002

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