Allow uploading of custom scene covers (#262)

* Refactor common code

* Further refactoring

* Add UI support for changing scene cover image

* Add backend support for changing scene screenshot

pkg/manager/scene_screenshot.go

@@ -0,0 +1,61 @@
+package manager
+
+import (
+	"bytes"
+	"image"
+	"image/jpeg"
+	"os"
+
+	"github.com/disintegration/imaging"
+
+	// needed to decode other image formats
+	_ "image/gif"
+	_ "image/png"
+)
+
+func writeImage(path string, imageData []byte) error {
+	f, err := os.Create(path)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	_, err = f.Write(imageData)
+	return err
+}
+
+func writeThumbnail(path string, thumbnail image.Image) error {
+	f, err := os.Create(path)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	return jpeg.Encode(f, thumbnail, nil)
+}
+
+func SetSceneScreenshot(checksum string, imageData []byte) error {
+	thumbPath := instance.Paths.Scene.GetThumbnailScreenshotPath(checksum)
+	normalPath := instance.Paths.Scene.GetScreenshotPath(checksum)
+
+	img, _, err := image.Decode(bytes.NewReader(imageData))
+	if err != nil {
+		return err
+	}
+
+	// resize to 320 width maintaining aspect ratio, for the thumbnail
+	const width = 320
+	origWidth := img.Bounds().Max.X
+	origHeight := img.Bounds().Max.Y
+	height := width / origWidth * origHeight
+
+	thumbnail := imaging.Resize(img, width, height, imaging.Lanczos)
+	err = writeThumbnail(thumbPath, thumbnail)
+	if err != nil {
+		return err
+	}
+
+	err = writeImage(normalPath, imageData)
+
+	return err
+}