Generate cover image (#376)

* Make mutating metadata ops mutation
* Implement scene generate screenshot
* Remove fetch policy on metadata mutations
* Port UI changes to v2.5
* Set generated image in database

pkg/manager/screenshot.go

@@ -0,0 +1,16 @@
+package manager
+
+import (
+	"github.com/stashapp/stash/pkg/ffmpeg"
+)
+
+func makeScreenshot(probeResult ffmpeg.VideoFile, outputPath string, quality int, width int, time float64) {
+	encoder := ffmpeg.NewEncoder(instance.FFMPEGPath)
+	options := ffmpeg.ScreenshotOptions{
+		OutputPath: outputPath,
+		Quality:    quality,
+		Time:       time,
+		Width:      width,
+	}
+	encoder.Screenshot(probeResult, options)
+}