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/internal/key.go

@@ -0,0 +1,61 @@
+// 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 internal
+
+import (
+	"strconv"
+
+	bp "github.com/gohugoio/hugo/bufferpool"
+
+	"github.com/mitchellh/hashstructure"
+)
+
+// ResourceTransformationKey are provided by the different transformation implementations.
+// It identifies the transformation (name) and its configuration (elements).
+// We combine this in a chain with the rest of the transformations
+// with the target filename and a content hash of the origin to use as cache key.
+type ResourceTransformationKey struct {
+	Name     string
+	elements []interface{}
+}
+
+// NewResourceTransformationKey creates a new ResourceTransformationKey from the transformation
+// name and elements. We will create a 64 bit FNV hash from the elements, which when combined
+// with the other key elements should be unique for all practical applications.
+func NewResourceTransformationKey(name string, elements ...interface{}) ResourceTransformationKey {
+	return ResourceTransformationKey{Name: name, elements: elements}
+}
+
+// Value returns the Key as a string.
+// Do not change this without good reasons.
+func (k ResourceTransformationKey) Value() string {
+	if len(k.elements) == 0 {
+		return k.Name
+	}
+
+	sb := bp.GetBuffer()
+	defer bp.PutBuffer(sb)
+
+	sb.WriteString(k.Name)
+	for _, element := range k.elements {
+		hash, err := hashstructure.Hash(element, nil)
+		if err != nil {
+			panic(err)
+		}
+		sb.WriteString("_")
+		sb.WriteString(strconv.FormatUint(hash, 10))
+	}
+
+	return sb.String()
+}