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/integrity/integrity_test.go

@@ -14,9 +14,13 @@
 package integrity
 
 import (
+	"html/template"
 	"testing"
 
+	"github.com/gohugoio/hugo/resources/resource"
+
 	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/resources/resource_transformers/htesting"
 )
 
 func TestHashFromAlgo(t *testing.T) {
@@ -46,3 +50,23 @@ func TestHashFromAlgo(t *testing.T) {
 		})
 	}
 }
+
+func TestTransform(t *testing.T) {
+	c := qt.New(t)
+
+	spec, err := htesting.NewTestResourceSpec()
+	c.Assert(err, qt.IsNil)
+	client := New(spec)
+
+	r, err := htesting.NewResourceTransformerForSpec(spec, "hugo.txt", "Hugo Rocks!")
+	c.Assert(err, qt.IsNil)
+
+	transformed, err := client.Fingerprint(r, "")
+
+	c.Assert(err, qt.IsNil)
+	c.Assert(transformed.RelPermalink(), qt.Equals, "/hugo.a5ad1c6961214a55de53c1ce6e60d27b6b761f54851fa65e33066460dfa6a0db.txt")
+	c.Assert(transformed.Data(), qt.DeepEquals, map[string]interface{}{"Integrity": template.HTMLAttr("sha256-pa0caWEhSlXeU8HObmDSe2t2H1SFH6ZeMwZkYN+moNs=")})
+	content, err := transformed.(resource.ContentProvider).Content()
+	c.Assert(err, qt.IsNil)
+	c.Assert(content, qt.Equals, "Hugo Rocks!")
+}