[Impeller] Dilated saveLayer content disappears when its source is outside the clip #195666

When a saveLayer uses a dilate image filter, Impeller drops content that is outside the parent clip even when the dilated result should extend into the visible area.

For example, on a 160 × 100 canvas:

• Draw a black background.
• Clip to (20, 20, 100, 60) in (x, y, width, height) notation.
• Save a layer with a dilation radius of (10, 10).
• Draw a white rectangle at (15, 45, 4, 10) and restore the layer.

The source rectangle is entirely to the left of the clip. After dilation, however, its right edge extends to x = 29, so a white strip should be visible inside the clip. Instead, the rectangle disappears.

This engine test reproduces the problem:

TEST_P(DlGoldenTest, DilatedContentOutsideClip) {
  SetWindowSize(impeller::ISize(160, 100));
  DisplayListBuilder builder;
  builder.DrawColor(DlColor::kBlack(), DlBlendMode::kSrc);
  builder.ClipRect(DlRect::MakeXYWH(20, 20, 100, 60));

  DlPaint layer_paint;
  layer_paint.setImageFilter(DlImageFilter::MakeDilate(10, 10));
  builder.SaveLayer(DlRect::MakeXYWH(0, 0, 160, 100), &layer_paint);
  // The rectangle is outside the clip, but its dilated right edge is inside.
  builder.DrawRect(DlRect::MakeXYWH(15, 45, 4, 10),
                   DlPaint().setColor(DlColor::kWhite()));
  builder.Restore();

  auto screenshot = MakeScreenshot(builder.Build());
  if (!screenshot) {
    GTEST_SKIP() << "making screenshots not supported.";
  }
  const uint8_t* row =
      screenshot->GetBytes() + 50 * screenshot->GetBytesPerRow();
  for (size_t channel = 0; channel < 3; channel++) {
    EXPECT_EQ(row[24 * 4 + channel], 255);
    EXPECT_EQ(row[15 * 4 + channel], 0);
  }
}

Pixel (24, 50) should be white because it is inside both the clip and the dilated rectangle. It is black with Impeller, and the test fails on both Metal and MetalSDF. Pixel (15, 50) is outside the clip and should remain black.

The same example renders as expected with Skia on both CPU and GPU:
https://fiddle.skia.org/c/86acbc9f1ca0a0a85db41a288e858498

Reproduced in native engine tests at revision 666d2c4e831d40738b8692464d57b4aaa9478acd, using host_debug_unopt_arm64 on macOS.