resources/images: Create padding image filter

Closes #11599

resources/images/padding.go

@@ -0,0 +1,50 @@
+// Copyright 2023 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 (
+	"image"
+	"image/color"
+	"image/draw"
+
+	"github.com/disintegration/gift"
+)
+
+var _ gift.Filter = (*paddingFilter)(nil)
+
+type paddingFilter struct {
+	top, right, bottom, left int
+	ccolor                   color.Color // canvas color
+}
+
+func (f paddingFilter) Draw(dst draw.Image, src image.Image, options *gift.Options) {
+	w := src.Bounds().Dx() + f.left + f.right
+	h := src.Bounds().Dy() + f.top + f.bottom
+
+	if w < 1 {
+		panic("final image width will be less than 1 pixel: check padding values")
+	}
+	if h < 1 {
+		panic("final image height will be less than 1 pixel: check padding values")
+	}
+
+	i := image.NewRGBA(image.Rect(0, 0, w, h))
+	draw.Draw(i, i.Bounds(), image.NewUniform(f.ccolor), image.Point{}, draw.Src)
+	gift.New().Draw(dst, i)
+	gift.New().DrawAt(dst, src, image.Pt(f.left, f.top), gift.OverOperator)
+}
+
+func (f paddingFilter) Bounds(srcBounds image.Rectangle) image.Rectangle {
+	return image.Rect(0, 0, srcBounds.Dx()+f.left+f.right, srcBounds.Dy()+f.top+f.bottom)
+}