buffered: Use the pending pixels when possible at At

(*Image).At can be unnecessarily slow since this tries to get
pixels from GPU. This change reduces the chance to read GPU by
using its pending pixels when possible.

Fixes #1137

image_test.go

@@ -1996,3 +1996,20 @@ func TestImageDrawTrianglesWithColorM(t *testing.T) {
 		}
 	}
 }
+
+// Issue #1137
+func TestImageDrawOver(t *testing.T) {
+	dst, _ := NewImage(320, 240, FilterDefault)
+	src := image.NewUniform(color.RGBA{0xff, 0, 0, 0xff})
+	// This must not cause infinite-loop.
+	draw.Draw(dst, dst.Bounds(), src, image.ZP, draw.Over)
+	for j := 0; j < 240; j++ {
+		for i := 0; i < 320; i++ {
+			got := dst.At(i, j)
+			want := color.RGBA{0xff, 0, 0, 0xff}
+			if got != want {
+				t.Errorf("At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+}

internal/buffered/image.go

@@ -127,7 +127,10 @@ func (i *Image) At(x, y int) (r, g, b, a byte, err error) {
 	if needsToDelayCommands {
 		panic("buffered: the command queue is not available yet at At")
 	}
-	// TODO: Use pending pixels
+	if i.pixels != nil {
+		idx := i.width*y + x
+		return i.pixels[4*idx], i.pixels[4*idx+1], i.pixels[4*idx+2], i.pixels[4*idx+3], nil
+	}
 	i.resolvePendingPixels(true)
 	return i.img.At(x, y)
 }