Allow slices in the image Filter funcs, not just varargs

[ci skip]

See #6255

resources/images/filters_test.go

@@ -0,0 +1,34 @@
+// 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 (
+	"testing"
+
+	"github.com/gohugoio/hugo/resources/internal"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestFilterHash(t *testing.T) {
+	c := qt.New(t)
+
+	f := &Filters{}
+
+	c.Assert(internal.HashString(f.Grayscale()), qt.Equals, internal.HashString(f.Grayscale()))
+	c.Assert(internal.HashString(f.Grayscale()), qt.Not(qt.Equals), internal.HashString(f.Invert()))
+	c.Assert(internal.HashString(f.Gamma(32)), qt.Not(qt.Equals), internal.HashString(f.Gamma(33)))
+	c.Assert(internal.HashString(f.Gamma(32)), qt.Equals, internal.HashString(f.Gamma(32)))
+
+}

resources/images/image.go

@@ -14,6 +14,7 @@
 package images
 
 import (
+	"fmt"
 	"image"
 	"image/color"
 	"image/gif"
@@ -259,3 +260,20 @@ func imageConfigFromImage(img image.Image) image.Config {
 	b := img.Bounds()
 	return image.Config{Width: b.Max.X, Height: b.Max.Y}
 }
+
+func ToFilters(in interface{}) []gift.Filter {
+	switch v := in.(type) {
+	case []gift.Filter:
+		return v
+	case []filter:
+		vv := make([]gift.Filter, len(v))
+		for i, f := range v {
+			vv[i] = f
+		}
+		return vv
+	case gift.Filter:
+		return []gift.Filter{v}
+	default:
+		panic(fmt.Sprintf("%T is not an image filter", in))
+	}
+}