Image resource refactor

This commit pulls most of the image related logic into its own package, to make it easier to reason about and extend.

This is also a rewrite of the transformation logic used in Hugo Pipes, mostly to allow constructs like the one below:

    {{ ($myimg | fingerprint ).Width }}

Fixes #5903
Fixes #6234
Fixes #6266

resources/images/smartcrop.go

@@ -0,0 +1,75 @@
+// Copyright 2019 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"
+
+	"github.com/disintegration/imaging"
+	"github.com/muesli/smartcrop"
+)
+
+const (
+	// Do not change.
+	// TODO(bep) image unexport
+	SmartCropIdentifier = "smart"
+
+	// This is just a increment, starting on 1. If Smart Crop improves its cropping, we
+	// need a way to trigger a re-generation of the crops in the wild, so increment this.
+	smartCropVersionNumber = 1
+)
+
+func newSmartCropAnalyzer(filter imaging.ResampleFilter) smartcrop.Analyzer {
+	return smartcrop.NewAnalyzer(imagingResizer{filter: filter})
+}
+
+// Needed by smartcrop
+type imagingResizer struct {
+	filter imaging.ResampleFilter
+}
+
+func (r imagingResizer) Resize(img image.Image, width, height uint) image.Image {
+	return imaging.Resize(img, int(width), int(height), r.filter)
+}
+
+func smartCrop(img image.Image, width, height int, anchor imaging.Anchor, filter imaging.ResampleFilter) (*image.NRGBA, error) {
+	if width <= 0 || height <= 0 {
+		return &image.NRGBA{}, nil
+	}
+
+	srcBounds := img.Bounds()
+	srcW := srcBounds.Dx()
+	srcH := srcBounds.Dy()
+
+	if srcW <= 0 || srcH <= 0 {
+		return &image.NRGBA{}, nil
+	}
+
+	if srcW == width && srcH == height {
+		return imaging.Clone(img), nil
+	}
+
+	smart := newSmartCropAnalyzer(filter)
+
+	rect, err := smart.FindBestCrop(img, width, height)
+	if err != nil {
+		return nil, err
+	}
+
+	b := img.Bounds().Intersect(rect)
+
+	cropped := imaging.Crop(img, b)
+
+	return imaging.Resize(cropped, width, height, filter), nil
+}