Add images.Process filter

This allows for constructs like:

```
{{ $filters := slice (images.GaussianBlur 8) (images.Grayscale) (images.Process "jpg q30 resize 200x") }}
{{ $img = $img | images.Filter $filters }}
```

Note that the `action` option in `images.Process` is optional (`resize` in the example above), so you can use the above to just set the target format, e.g.:

```
{{ $filters := slice (images.GaussianBlur 8) (images.Grayscale) (images.Process "jpg") }}
{{ $img = $img | images.Filter $filters }}
```

Fixes #8439

resources/images/filters.go

@@ -30,6 +30,15 @@ const filterAPIVersion = 0
 
 type Filters struct{}
 
+func (*Filters) Process(spec any) gift.Filter {
+	return filter{
+		Options: newFilterOpts(spec),
+		Filter: processFilter{
+			spec: cast.ToString(spec),
+		},
+	}
+}
+
 // Overlay creates a filter that overlays src at position x y.
 func (*Filters) Overlay(src ImageSource, x, y any) gift.Filter {
 	return filter{

resources/images/image.go

@@ -201,7 +201,7 @@ func (p *ImageProcessor) DecodeExif(r io.Reader) (*exif.ExifInfo, error) {
 	return p.exifDecoder.Decode(r)
 }
 
-func (p *ImageProcessor) ApplyFiltersFromConfig(src image.Image, conf ImageConfig) (image.Image, error) {
+func (p *ImageProcessor) FiltersFromConfig(src image.Image, conf ImageConfig) ([]gift.Filter, error) {
 	var filters []gift.Filter
 
 	if conf.Rotate != 0 {
@@ -246,6 +246,14 @@ func (p *ImageProcessor) ApplyFiltersFromConfig(src image.Image, conf ImageConfi
 	default:
 
 	}
+	return filters, nil
+}
+
+func (p *ImageProcessor) ApplyFiltersFromConfig(src image.Image, conf ImageConfig) (image.Image, error) {
+	filters, err := p.FiltersFromConfig(src, conf)
+	if err != nil {
+		return nil, err
+	}
 
 	if len(filters) == 0 {
 		return p.resolveSrc(src, conf.TargetFormat), nil
@@ -396,6 +404,15 @@ func imageConfigFromImage(img image.Image) image.Config {
 	return image.Config{Width: b.Max.X, Height: b.Max.Y}
 }
 
+// UnwrapFilter unwraps the given filter if it is a filter wrapper.
+func UnwrapFilter(in gift.Filter) gift.Filter {
+	if f, ok := in.(filter); ok {
+		return f.Filter
+	}
+	return in
+}
+
+// ToFilters converts the given input to a slice of gift.Filter.
 func ToFilters(in any) []gift.Filter {
 	switch v := in.(type) {
 	case []gift.Filter:

resources/images/process.go

@@ -0,0 +1,43 @@
+// Copyright 2023 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"
+	"image/draw"
+
+	"github.com/disintegration/gift"
+)
+
+var _ ImageProcessSpecProvider = (*processFilter)(nil)
+
+type ImageProcessSpecProvider interface {
+	ImageProcessSpec() string
+}
+
+type processFilter struct {
+	spec string
+}
+
+func (f processFilter) Draw(dst draw.Image, src image.Image, options *gift.Options) {
+	panic("not supported")
+}
+
+func (f processFilter) Bounds(srcBounds image.Rectangle) image.Rectangle {
+	panic("not supported")
+}
+
+func (f processFilter) ImageProcessSpec() string {
+	return f.spec
+}