graphics: Bug fix: Too small mipmap

Bug: #839

image.go

@@ -325,6 +325,17 @@ func (i *Image) drawImage(img *Image, options *DrawImageOptions) {
 		if level < 0 {
 			panic(fmt.Sprintf("ebiten: level must be >= 0 but %d", level))
 		}
+
+		// If the image can be scaled into 0 size, adjust the level. (#839)
+		w, h := bounds.Dx(), bounds.Dy()
+		for level >= 0 {
+			s := 1 << uint(level)
+			if w/s == 0 || h/s == 0 {
+				level--
+				continue
+			}
+			break
+		}
 	}
 	if level > 6 {
 		level = 6

image_test.go

@@ -1652,3 +1652,21 @@ func TestImageSubImageSubImage(t *testing.T) {
 		}
 	}
 }
+
+// Issue 839
+func TestImageTooSmallMipmap(t *testing.T) {
+	const w, h = 16, 16
+	src, _ := NewImage(w, h, FilterDefault)
+	dst, _ := NewImage(w, h, FilterDefault)
+
+	src.Fill(color.White)
+	op := &DrawImageOptions{}
+	op.GeoM.Scale(1, 0.24)
+	op.Filter = FilterLinear
+	dst.DrawImage(src.SubImage(image.Rect(5, 0, 6, 16)).(*Image), op)
+	got := dst.At(0, 0).(color.RGBA)
+	want := color.RGBA{0xff, 0xff, 0xff, 0xff}
+	if got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+}