resources/images: Allow to set background fill colour

Closes #6298

resources/images/color.go

@@ -0,0 +1,85 @@
+// 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 (
+	"encoding/hex"
+	"image/color"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+// AddColorToPalette adds c as the first color in p if not already there.
+// Note that it does no additional checks, so callers must make sure
+// that the palette is valid for the relevant format.
+func AddColorToPalette(c color.Color, p color.Palette) color.Palette {
+	var found bool
+	for _, cc := range p {
+		if c == cc {
+			found = true
+			break
+		}
+	}
+
+	if !found {
+		p = append(color.Palette{c}, p...)
+	}
+
+	return p
+}
+
+// ReplaceColorInPalette will replace the color in palette p closest to c in Euclidean
+// R,G,B,A space with c.
+func ReplaceColorInPalette(c color.Color, p color.Palette) {
+	p[p.Index(c)] = c
+}
+
+func hexStringToColor(s string) (color.Color, error) {
+	s = strings.TrimPrefix(s, "#")
+
+	if len(s) != 3 && len(s) != 6 {
+		return nil, errors.Errorf("invalid color code: %q", s)
+	}
+
+	s = strings.ToLower(s)
+
+	if len(s) == 3 {
+		var v string
+		for _, r := range s {
+			v += string(r) + string(r)
+		}
+		s = v
+	}
+
+	// Standard colors.
+	if s == "ffffff" {
+		return color.White, nil
+	}
+
+	if s == "000000" {
+		return color.Black, nil
+	}
+
+	// Set Alfa to white.
+	s += "ff"
+
+	b, err := hex.DecodeString(s)
+	if err != nil {
+		return nil, err
+	}
+
+	return color.RGBA{b[0], b[1], b[2], b[3]}, nil
+
+}