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
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1735773003

Review URL: https://codereview.chromium.org/1735773003

src/effects/gradients/Sk4fGradientBase.cpp

@@ -207,8 +207,9 @@ GradientShaderBase4fContext::GradientShaderBase4fContext(const SkGradientShaderB
                                 clamp_color, clamp_pos,
                                 componentScale);
     } else if (shader.fTileMode == SkShader::kMirror_TileMode) {
+        const int count = fIntervals.count();
         // synthetic flipped intervals in [1 .. 2)
-        for (int i = fIntervals.count() - 1; i >= 0; --i) {
+        for (int i = count - 1; i >= 0; --i) {
             const Interval& interval = fIntervals[i];
             const SkScalar p0 = interval.fP0;
             const SkScalar p1 = interval.fP1;
@@ -216,6 +217,19 @@ GradientShaderBase4fContext::GradientShaderBase4fContext(const SkGradientShaderB
             Sk4f c = Sk4f::Load(interval.fC0.fVec) + dc * Sk4f(p1 - p0);
             fIntervals.emplace_back(c, dc * Sk4f(-1), 2 - p1, 2 - p0);
         }
+
+        if (!dx_is_pos) {
+            // When dx is negative, our initial invervals are in (1..0] order.
+            // The loop above appends their flipped counterparts, pivoted in 2: (1..0](2..1]
+            // To achieve the expected monotonic interval order, we need to
+            // swap the two halves: (2..1](1..0]
+            // TODO: we can probably avoid this late swap with some additional logic during
+            //       the initial interval buildup.
+            SkASSERT(fIntervals.count() == count * 2)
+            for (int i = 0; i < count; ++i) {
+                SkTSwap(fIntervals[i], fIntervals[count + i]);
+            }
+        }
     }
 
     SkASSERT(fIntervals.count() > 0);