abstract jpeg encoding away from "WSServer"

Additionally make thumbnail encoding threadpooled as well, just so it (probably) doesn't block as much.

cvmts/src/ThumbnailJPEGEncoderWorker.ts

@@ -0,0 +1,31 @@
+import { Size } from '@cvmts/shared';
+import Piscina from 'piscina';
+import sharp from 'sharp';
+
+const kThumbnailSize: Size = {
+	width: 400,
+	height: 300
+};
+
+// this returns appropiate Sharp options to deal with CVMTS raw framebuffers
+// (which are RGBA bitmaps, essentially. We probably should abstract that out but
+// that'd mean having to introduce that to rfb and oihwekjtgferklds;./tghnredsltg;erhds)
+function GetRawSharpOptions(size: Size): sharp.CreateRaw {
+	return {
+		width: size.width,
+		height: size.height,
+		channels: 4
+	};
+}
+
+export default async (opts: any) => {
+	let out = await sharp(opts.buffer, { raw: GetRawSharpOptions(opts.size) })
+		.resize(kThumbnailSize.width, kThumbnailSize.height, { fit: 'fill' })
+		.jpeg({
+			quality: opts.quality || 75
+		})
+		.toFormat('jpeg')
+		.toBuffer();
+
+	return Piscina.move(out);
+};