internal/atlas: optimize adjustDestinationPixel

```
go test -bench=BenchmarkAdjustPixel -run=^$ -count=5 ./internal/atlas/
```

```
name           old time/op  new time/op  delta
AdjustPixel-8  2.59ns ± 1%  2.12ns ± 1%  -18.16%  (p=0.008 n=5+5)
```

internal/atlas/image.go

@@ -17,7 +17,6 @@ package atlas
 import (
 	"fmt"
 	"image"
-	"math"
 	"runtime"
 	"sync"
 
@@ -805,7 +804,11 @@ func adjustDestinationPixel(x float32) float32 {
 	//     float32(math.Floor((float64(x)+1.0/6.0)*3) / 3)
 	//
 	// The actual implementation is more optimized than the above implementation.
-	ix := float32(math.Floor(float64(x)))
+	var ix float32
+	ix = float32(int(x))
+	if x < 0 && x != ix {
+		ix -= 1
+	}
 	frac := x - ix
 	switch {
 	case frac < 3.0/16.0:

internal/atlas/image_test.go

@@ -737,6 +737,40 @@ func TestImageWritePixelsModify(t *testing.T) {
 	}
 }
 
+func TestAdjustPixel(t *testing.T) {
+	tests := []struct {
+		X     float32
+		Y     float32
+		Delta float32
+	}{
+		{
+			X:     -0.1,
+			Y:     0.9,
+			Delta: 1,
+		},
+		{
+			X:     -1,
+			Y:     0,
+			Delta: 1,
+		},
+		{
+			X:     -1.9,
+			Y:     1.1,
+			Delta: 3,
+		},
+		{
+			X:     -2,
+			Y:     1,
+			Delta: 3,
+		},
+	}
+	for _, tc := range tests {
+		if rx, ry := atlas.AdjustDestinationPixelForTesting(tc.X)+tc.Delta, atlas.AdjustDestinationPixelForTesting(tc.Y); rx != ry {
+			t.Errorf("adjustDestinationPixel(%f) + 1 must equal to adjustDestinationPixel(%f) but not (%f vs %f)", tc.X, tc.Y, rx, ry)
+		}
+	}
+}
+
 func BenchmarkAdjustPixel(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		atlas.AdjustDestinationPixelForTesting(float32(i) / 17)