images: Add images.Overlay filter

This allows for constructs ala:

```
{{ $overlay := $img.Filter (images.Overlay $logo 50 50 )}}
```
Or:

```
{{ $logoFilter := (images.Overlay $logo 50 50 ) }}
{{ $overlay := $img | images.Filter $logoFilter }}
```

Which will overlay the logo in the top left corner (x=50, y=50) of `$img`.

Fixes #8057
Fixes #4595
Updates #6731

resources/images/overlay.go

@@ -0,0 +1,43 @@
+// Copyright 2020 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package images
+
+import (
+	"fmt"
+	"image"
+	"image/draw"
+
+	"github.com/disintegration/gift"
+)
+
+var _ gift.Filter = (*overlayFilter)(nil)
+
+type overlayFilter struct {
+	src  ImageSource
+	x, y int
+}
+
+func (f overlayFilter) Draw(dst draw.Image, src image.Image, options *gift.Options) {
+	overlaySrc, err := f.src.DecodeImage()
+	if err != nil {
+		panic(fmt.Sprintf("failed to decode image: %s", err))
+	}
+
+	gift.New().Draw(dst, src)
+	gift.New().DrawAt(dst, overlaySrc, image.Pt(f.x, f.y), gift.OverOperator)
+}
+
+func (f overlayFilter) Bounds(srcBounds image.Rectangle) image.Rectangle {
+	return image.Rect(0, 0, srcBounds.Dx(), srcBounds.Dy())
+}