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/resource_transformers/htesting/testhelpers.go

@@ -0,0 +1,80 @@
+// 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 htesting
+
+import (
+	"path/filepath"
+
+	"github.com/gohugoio/hugo/cache/filecache"
+	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/hugofs"
+	"github.com/gohugoio/hugo/media"
+	"github.com/gohugoio/hugo/output"
+	"github.com/gohugoio/hugo/resources"
+	"github.com/spf13/afero"
+	"github.com/spf13/viper"
+)
+
+func NewTestResourceSpec() (*resources.Spec, error) {
+	cfg := viper.New()
+	cfg.Set("baseURL", "https://example.org")
+	cfg.Set("publishDir", "public")
+
+	imagingCfg := map[string]interface{}{
+		"resampleFilter": "linear",
+		"quality":        68,
+		"anchor":         "left",
+	}
+
+	cfg.Set("imaging", imagingCfg)
+
+	fs := hugofs.NewMem(cfg)
+
+	s, err := helpers.NewPathSpec(fs, cfg, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	filecaches, err := filecache.NewCaches(s)
+	if err != nil {
+		return nil, err
+	}
+
+	spec, err := resources.NewSpec(s, filecaches, nil, output.DefaultFormats, media.DefaultTypes)
+	return spec, err
+}
+
+func NewResourceTransformer(filename, content string) (resources.ResourceTransformer, error) {
+	spec, err := NewTestResourceSpec()
+	if err != nil {
+		return nil, err
+	}
+	return NewResourceTransformerForSpec(spec, filename, content)
+}
+
+func NewResourceTransformerForSpec(spec *resources.Spec, filename, content string) (resources.ResourceTransformer, error) {
+	filename = filepath.FromSlash(filename)
+
+	fs := spec.Fs.Source
+	if err := afero.WriteFile(fs, filename, []byte(content), 0777); err != nil {
+		return nil, err
+	}
+
+	r, err := spec.New(resources.ResourceSourceDescriptor{Fs: fs, SourceFilename: filename})
+	if err != nil {
+		return nil, err
+	}
+
+	return r.(resources.ResourceTransformer), nil
+}