SkMatrix::preScale() is too conservative

SkMatrix::preScale() always sets the kScale bit, which means something
like

  m = SkMatrix::MakeScale(2, 2);
  m.preScale(0.5, 0.5);

leaves m.getType() == kScale_Mask, and can throw off the bitmap proc
heuristics.

We could detect the inverse case and clear the scale bit instead.

R=reed@google.com,caryclark@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1531323002

Review URL: https://codereview.chromium.org/1531323002
1 file changed