4f gradient negative-dx interval fixup for kMirror_TileMode

4f intervals are meant to be monotonically increasing in X.  This means
we invert the point order when dx < 0:

  [0...1) -> (1...0]

For kMirror_TileMode, we also append a duplicate/flipped interval
sequence and expand the range to 2:

  [0...1) -> [0...1)[1...2)

When dx < 0, the current logic yields

  (1...0] -> (1...0](2...1]

which violates the interval monotonicity.

To achieve the proper order, we need to swap the two halves:

  (1...0] -> (1...0](2...1] -> (2...1](1...0]

R=reed@google.com
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Review URL: https://codereview.chromium.org/1735773003

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