Implement fallback images in a similar fashion to watermarks. (#374)

* 220. Implement fallback images in a similar fashion to watermarks.

* 220. Fix error formatting.

* 220. Fixes based on @darthsim's feedback.

image_data.go

@@ -0,0 +1,90 @@
+package main
+
+import (
+	"bytes"
+	"encoding/base64"
+	"fmt"
+	"os"
+)
+
+func getWatermarkData() (*imageData, error) {
+	if len(conf.WatermarkData) > 0 {
+		return base64ImageData(conf.WatermarkData)
+	}
+
+	if len(conf.WatermarkPath) > 0 {
+		return fileImageData(conf.WatermarkPath)
+	}
+
+	if len(conf.WatermarkURL) > 0 {
+		return remoteImageData(conf.WatermarkURL)
+	}
+
+	return nil, nil
+}
+
+func getFallbackImageData() (*imageData, error) {
+	if len(conf.FallbackImageData) > 0 {
+		return base64ImageData(conf.FallbackImageData)
+	}
+
+	if len(conf.FallbackImagePath) > 0 {
+		return fileImageData(conf.FallbackImagePath)
+	}
+
+	if len(conf.FallbackImageURL) > 0 {
+		return remoteImageData(conf.FallbackImageURL)
+	}
+
+	return nil, nil
+}
+
+func base64ImageData(encoded string) (*imageData, error) {
+	data, err := base64.StdEncoding.DecodeString(encoded)
+	if err != nil {
+		return nil, fmt.Errorf("Can't decode image data: %s", err)
+	}
+
+	imgtype, err := checkTypeAndDimensions(bytes.NewReader(data))
+	if err != nil {
+		return nil, fmt.Errorf("Can't decode image: %s", err)
+	}
+
+	return &imageData{Data: data, Type: imgtype}, nil
+}
+
+func fileImageData(path string) (*imageData, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("Can't read watermark: %s", err)
+	}
+
+	fi, err := f.Stat()
+	if err != nil {
+		return nil, fmt.Errorf("Can't read watermark: %s", err)
+	}
+
+	imgdata, err := readAndCheckImage(f, int(fi.Size()))
+	if err != nil {
+		return nil, fmt.Errorf("Can't read watermark: %s", err)
+	}
+
+	return imgdata, err
+}
+
+func remoteImageData(imageURL string) (*imageData, error) {
+	res, err := requestImage(imageURL)
+	if res != nil {
+		defer res.Body.Close()
+	}
+	if err != nil {
+		return nil, fmt.Errorf("Can't download image: %s", err)
+	}
+
+	imgdata, err := readAndCheckImage(res.Body, int(res.ContentLength))
+	if err != nil {
+		return nil, fmt.Errorf("Can't download image: %s", err)
+	}
+
+	return imgdata, err
+}